So... OK, I just finally found out how I can play this on fullscreen without stretching...
...I was looking for it too complicated. I never realized that it was all in my monitor's settings, actually: the Image Size was set to 'Wide'. I simply set it to 'Auto'. So everytime I fullscreen anything that's 4:3 now, it appears as 4:3 like it should, so :3
Seeing this I tried to play Touhou like a confident little kid...
...and of course I got owned. Playing in Windowed for 2 months has gotten me used to looking at really small things.
But I persisted and tried and eventually ended up 1CC'ing TH9 on Normal (for the second time, without an insane amount of luck like my first 1CC) using Marisa. I ended up with a happy 87 million points (though I must admit it could've been larger if I didn't stupid up my third round against Shiki), and was able to add another ending.txt into my collection, and tried to save the replay, when I completely forgot about the Replay glitch and the replay didn't save :\ But still, at least I got Ending No.2's script wrote down~
...yeah as you may have suspected that's what's in my sig.