CRJ-200 First Officer reported performing an incorrect sequence in transferring bleeds with an aircraft that had a list of MEL items that resulted in an unpressurized cabin without air conditioning. The cabin temperature became unsafe for the passengers and flight crew and the flight returned to the departure airport.

Narrative

The Captain and I received an aircraft with a list of MEL items. One item was the right air conditioning pack was out of service. Along with the limitations we discussed; we also did all of our research in a briefing and covered all the special procedures associated with transferring the bleeds; which required leaving the right engine source bleed valve and right pack closed the entire flight. It also required an unusual sequence of transferring the remaining 3 bleeds buttons that swept from right to left in continuous sequence. Also to note; N1 values during the transfer were to be reduced to 60 percent if we were unable to level off to avoid single pack over-pressurization. If you are able to level off and cruise; you did not have to reduce to 60% N1. That and some other complexities seemed a bit more complicated than normal so I asked the Captain to kind of cue me about which procedure he was going to use prior to the transfer so I would be on the same page.After a busy ZZZ departure; and with me fairly fresh off my initial operating experience training; I was assigned new headings; altitudes; and frequencies while still having to running the after-takeoff checklist as permitted. When I approach the part of my flow for transferring the bleeds; I believe I said to the Captain something along the lines of 'I have to transfer the bleeds.' He was also busy with his flying duties and perhaps fairly occupied; but I believe he cued me to transfer the bleeds. With me on the radio and trying to accomplish the checklist and relying on his guidance to ensure I did it with his approval; I think there was a miscommunication or misunderstanding and before he realized this situation; I had already begun transferring the bleeds in the exact sequence we had discussed on the ground. However; thrust was not reduced and we had perhaps let that detail slip past us I speculate after debriefing with the Captain postflight which may have contributed to right pack over-pressurizing; which is what I was fearing would happen in a worst-case scenario. In fact; we even had the QRH open on the exact page to address a pack over-pressure prior to pushing from the gate which I'm glad we did. After transferring the bleeds; a left pack high pressure message appeared; to which we responded with the QRH book. After running the QRH; the pack did not reset or come back online. We decided we could not go all the way from ZZZ to ZZZ1 for 1.5 hours unpressurized and without air conditioning for both passengers and crew. We were vectored around while devising a plan and concurring with Dispatch after we ran the numbers for fuel burn and we determined we needed 30 minutes to burn off 2;000 lb. of fuel as we are over max landing weight. We deployed spoiler and landing gear and flew around in a high drag configuration which required higher thrust to address this issue. After beginning the process the first phone call from the Flight Attendant arrived. The cabin was getting hot and some passengers were getting restless and would like the plane to return to land. We expressed that we understood and were doing our best to burn off the fuel and work on getting back into the airport. Not long after; another phone call from the Flight Attendant; this one was an emergency phone call. A passenger with a heart condition was having some trouble breathing due to the heated up passenger cabin and we must land immediately. We ran the numbers again and figured we will be about 1;000 lb. overweight so we concurred with Dispatch and they sent me back a message saying be easy on the landing but do what we must. We had already [requested priority handling] to ATC as well. They vectored us to the runway of our choice and we called for the trucks and EMS on our arrival. A very busy return to the airport; I was getting ATIS; inputting landing setup data; I updated and loaded and cleaned up the approach in the FMS; while selecting new altitude assignments; while he selected headings;running in range checklist; a quick approach briefing; inputting navigation radio frequencies; we ran an approaching checklist while sometimes taking calls from both the Flight Attendant and ATC - it took both of us pilots as a team to accomplish all the tasks. Later on; on the ground; I discovered the Captain forgot to run [the] depressurization landing checklist but since ram air was open due to the pack MEL and we were low altitude; the aircraft fortunately never pressurized anyway. After cleared for the approach and descending; we learned suddenly as the Captain reduced power as we approached for landing; the aircraft gave us a jet pipe overheat fail message on our display but if we do the QRH; we will not be focused and stabilized on the approach - and put flying the airplane first as well as the now 2; emergency medical patients in the back. In fact we were informed claiming that second passenger has asthma; who was severely panicking; and also needed to land immediately. We must land now as safety permits; we cannot go around with the conditions in the back are getting worse by the second. All engine gauges are also indicated good readings; not to mention it's a detection loop gone bad; not an actual overheat warning. We didn't have time to run the QRH; I would've been reading it on the ground when I had to call Operations and write down ATC instructions which was unfortunate because I know we wanted to do the QRH and were trying to do the best we could.The Captain lands very smoothly and we are escorted by the trucks with ATC priority to the gate while I'm trying to get a gate assignment from Operations and I'm talking to ATC and my Captain is driving. A call from the Flight Attendant after landing confirms the guy with heart condition is doing somewhat decently okay now which is finally some relieving news; but we know definitely still need to get everyone off the plane because it's hot and more heat exposure we know will only make the passengers worse off. Getting passengers out was a little uncoordinated by Ramp and the gate agents. The Ramper didn't expect us and they gave us some resistance for a minute; trying to tell us we couldn't come in the gate; but Operations already cleared us for the gate and there were firetrucks everywhere so it was obvious this was not normal circumstances. We just kinda had to do our best to signal them we had to come in anyway to which eventually; but slowly; they adapted to in confusion. The critical passengers the fire department urged to come down the stairs after he exclaimed how hot it felt and the Captain simultaneously agreed because the cabin was way indeed too hot and dangerous back for them to stay. A third passengers also wanted off with her young infant which was understandable. The Gate Agent however; did not want us to deplane and there was a bit of somewhat stressful; uncoordinated flow to the deplaning process until they connected the bridge and finally all the non-critical passengers were able to deplane the aircraft.In the future; I think this experience will make me extremely vigilant and cautious transferring the bleeds in abnormal circumstances. Although; I intentionally paused prior to transferring them to ensure I critically thought through and tried to transfer correctly; that one possible detail could've over-pressurized the packs. It's unfortunate that it is very easy to do in this aircraft especially when you always are trained to do it one way and have never practiced or seen this new technique before but regardless it's critical we perform this correctly. I have definitely learned that attention to detail as a result of this as well as no matter what and how busy we are; we have to remember to also run the depressurized landing procedures if an unfortunate event similar to this ever happened again. The event has inspired me to review special procedures like reading up on unpressurized takeoffs and landings; as well as MELs to the packs; and to constantly review emergency memory items; because you never know what can happen. It's very unfortunate this happened but we are very grateful we were able to successfully return back to the airport and get our sick passengers off the plane in time where everyone made it back safely.