The door problem is likely due to the lever above the counter wheel not being down all the way. My suggestion is to learn to do the entire process inside a changing bag. It really isn't very hard (if I can do it, trust me on this, anybody can). I pre-clean 20 cartridges and put them (disassembled) in a soup bowl. I add a second bowl for the finished product. Then, without ever opening the bag, I load the film in the loader and go through the process of loading 18 cartridges counting 38 clicks per roll. I use Scotch Magic tape from a small disposable dispenser.
With a little practice, you can load an entire roll while watching a sit-com. Make sure you've used the bathroom before you start. ;>)