1. Why? While the aspect ratio is 4:3 all is fine.
If it’s not exactly a 4:3 aspect ratio without the scanline overlay it may look a bit scrappy, but with the scanlines all looks great.
2. Here’s a link to my dropbox. Here you can download the overlay.
3. You’re right. It depends on the emulator. PAL NES is always 240 lines. NTSC is 224 + 2 x 8 “hidden” lines = 240.
Currently i upload a video on Youtube.
I’ll post later a link.