Air carrier Captain reported returning to the departure airport after an MEL'd right high pressure shutoff valve caused several other pressurization failures.

Narrative

We reported on time for ZZZ-ZZZZ; the inbound aircraft arrived late. Upon arrival at the gate; we conferred with the deplaning Captain regarding the active MEL XX-XX-XXX (Right HP Shutoff Valve - Closed); which had been applied on the previous leg in ZZZZ1.My FO (First Officer) and I thoroughly reviewed and complied with the MEL; including all O and M procedures.Departure and initial climb were uneventful. While climbing through approximately FL200; we received a R BLEED OVHT caution. I was Pilot Flying (PF); my FO was Pilot Monitoring (PM) and initiated the ECL (Electronic Checklist). The ECL directs selecting R BLEED OFF; which was already selected off due to the MEL. No further ECL actions were applicable.Although compliant with MEL procedures; we were concerned about the overheat indication on a system that should have been isolated per MEL. With no effective guidance in the ECL and an active caution; I directed the FO to contact Maintenance Control via SAT phone while I continued flying and handled VHF comms.Maintenance Control advised manually closing the crossbleed valve to isolate the bleed system further. After closing the crossbleed; the OVHT indication persisted. This also disabled anti-ice protection on the right wing; a concern due to enroute weather and potential icing conditions ahead over Location A.While evaluating options with Maintenance and Dispatch; we received a R PACK FAIL caution; several minutes after closing the crossbleed valve. Despite the pack lacking a bleed air source; we ran the ECL to verify no relevant procedural guidance was missed. • With the R PACK selected OFF; duct pressure remained normal. • With the R PACK selected AUTO; duct pressure dropped (~9 PSI); and the PACK symbol remained amber. The caution returned. • This raised concern over a potential leaking valve.Shortly after; while level at FL300; we began experiencing strong; abnormal trim air surges into the cockpit; with duct temps reaching 194°F. These surges were more intense and erratic than typical when operating with MEL'd PRV (pressure relief valves). With cockpit temperatures rising rapidly and no guidance in the ECL; we became increasingly concerned about the escalating behavior in the single-bleed; single-pack configuration and potential further system degradation.My FO suggested turning Trim Air OFF; which I agreed to. This action immediately stopped the abnormal temperature surges and stabilized the cabin environment.Given the cumulative concerns -- persistent R BLEED OVHT; R PACK FAIL; abnormal trim air behavior; and no guidance available -- we decided along with Maintenance/Dispatch concurrence to return to ZZZ. We requested priority handling due to the current system anomalies and anticipated overweight landing. We requested and were cleared to descend to 10;000 ft as a precaution in case of further pressurization loss.At approximately 10;000 ft; the R BLEED OVHT caution cleared.We coordinated with ATC for a return to ZZZ; selecting Runway XXR due to its length and favorable wind alignment (winds 140 degrees). We completed the Overweight Landing ECL and briefed the Inflight crew and customers.I conducted an ILS XXR approach in visual conditions. The aircraft landed Flaps 5 with LO autobrakes; transitioning to light manual braking and maximum reverse. Touchdown was smooth (~150 FPM); well within the TDZ. Aircraft weight was ~141;000 lbs (VREF: 137 + 5). Brake temps were normal; highest at 05. Emergency equipment was staged as a precaution.We taxied to the gate uneventfully. CFR (Crash Fire Rescue) met us at the gate and confirmed no abnormal indications remained. No further assistance was needed.Logbook entries were made for the aircraft indications and overweight landing.