1) You may not need a changing bag. Since you have a bathroom with no windows it is much easier to just cover the edges of the doorway to make it light-tight. Use that for loading your film holders, and then later go back in to unload the exposed film and put it into a daylight developing tank.
Once the film is in the tank you can process the film under normal room light.
2) Your bathroom probably has an exhaust fan and hot and cold running water. I’d get a plastic bucket or something similar, with a flat bottom, and do my developing in that, rather than trying to get the developing tank to sit flat in my bathroom sink. Drips and splashes stay in the bucket until I flush them down the drain. If you can fit a small table in there, you’ll have a tiny, but functional workspace. That table might fit in a bathtub or shower stall.
3) 4x5 enlargers are rather expensive these days, mostly due to shipping costs. Instead of that, look for a ‘Graflarger back’. Remove the ground glass on your Crown Graphic and insert this device instead.
https://www.ebay.com/itm/364122263519?It is a fluorescent light and negative carrier which goes on the camera. The ballast and transformer are in a second ( heavier) unit that sits on the table. With the camera on a copy stand you have an enlarger. You can fabricate something similar with an LED screen and a negative carrier from a Besseler or Omega enlarger the negative carriers are aluminum and can be cut down to a smaller size. Either fluorescent or LED will run at a much cooler physical temperature than an incandescent bulb, but the color temperature of either of these is not compatible with multigrade enlarging paper.
If you want to fabricate an incandescent ‘head’ for your camera/enlarger I’d suggest starting with a large clamp light and reflector from a hardware store, and a PH 211 or PH212 enlarger bulb from B&H photo. The smaller 75 watt bulb should be plenty bright, and you can use this with the polycontrast/multicontrast filters and photopaper sold by Ilford.
4) With black and white film, any of these light sources make nice backlights for copying the negative onto a digital camera. If you use color film then use either a tungsten bulb or an LED panel which has a very high CRI ( Color Rendering Index) of 95 or higher. The graflarger back isn’t meant for color.
5) Look for a 120 roll film back for your press camera. These come in 3 sizes: 6x6 (12 exposures), 6x7 (10 exposures), and 6x9 cm (8 exposures per roll. You can load a fresh roll of film in daylight and process it alongside the film from your Rollei.
6) Yes. UHH has both a film area and a Black and white section. The latter deals with both film and digital originals.
Enjoy your return to the darkroom
1) You may not need a changing bag. Since you have... (