Air carrier Captain reported after landing; another aircraft taxied onto the end of the runway; blocking the carrier's exit. Carrier exited and observed a near miss between that aircraft and another that departed opposite direction.

Narrative

This was an OE flight and my first leg as pilot flying (PF) in the left seat the Line Check Pilot (LCP) was pilot monitoring (PM) and handling radio calls; making frequent position reports. ZZZ was overloaded with traffic. There were multiple GA aircraft and helicopters all trying to use the field. After taking an extended vector to allow other traffic in; we were given an approach clearance for Runway XX. The winds were a direct crosswind gusting to 25 kts. We had planned to roll to the end to spare the brakes any excess heat so we could make the turn as quick as possible. While rolling to the end; I had noticed there was a taildragging aircraft sitting there so we had nowhere to go. We were stopped on the runway waiting for him to move out of the way; and in our minds this would have entailed taxiing to the departure end in use (XX) and instead this individual proceeded to taxi onto the runway (XY) without making any radio calls and hugging the end of the pavement to make room. During this process; I heard another aircraft getting ready to takeoff of Runway XX while we were still holding for the small aircraft to clear. Over the radio we announced we were still on the runway. At this point there may have been three aircraft on the runway but this is speculation. Once able to taxi onto the taxiway and clear the runway; the LCP said 'why dont you use Runway XX so its easier on all of us' which was met with the response: 'what are you an air traffic controller?'. This individual also mentioned something about a maintenance run up or flight. We tried to get a tail number but at this point we were unable. I then hear the aircraft on the departure end on XX announce their takeoff. This means they either had to takeoff over this individual or if he decided to takeoff without announcing it; potentially had a near miss event. After this we taxi to the gate; shutdown and deplane. It is our agreed opinions this individual compromised the safety of multiple aircraft and lives with irresponsible airmanship and a dangerous attitude. Cause: Dangerous and unprofessional airman.