Is my source upscaled? (And a few questions about upscaling)

[OvejaNegra]

I'm doing an encode of my Ghost in the shell BD (the movie from 1995).

And i was little dissapointed with the detail, when i found this:

https://forum.doom9.org/showthread.php?t=170832


So i decided to try:

DebilinearM(1280,720,showmask=0,kernel="bicubic",cubic=true)

and to my surprise, the mask (supposedly) reveals that the source seems to be 1280x720. The only places where the mask reveals higher resolution detail is:

-On the credits
-Lights glows (seems to be done on post production, even histrogram() on avisynth shows that those pixels are outside the 16-235 luma range).

-Text on computer's screens (maybe donde with post production).

¿Should i conclude that the source master is 720p and it was upscaled on post production?

i did a simple test using

ups = last .Debicubic(1280, 720).BicubicResize(1920, 1080).Subtitle("ups", x=8, y=0)

Interleave(last,ups)

And i can't see any difference! Even the grain stays on the same place!

I espected GITS to be scanned from film to full 1080p. But maybe they used an old master.
Or maybe im talking nonsense!

I also did this:

Debicubic(1280, 720)
nnedi3_rpow2(rfactor=2, nsize=0, nns=3, qual=1, etype=0, pscrn=1, threads=0, opt=0, fapprox=15,cshift="spline36resize",fwidth=1920,fheight=1080)

And the lines look better on my eyes. Should i leave it like that?

Can anybody post an example of how use DebilinearM to extract / protect/mask the 1080 pixels and import them over the
nnedi final image?

Should i do all this on 16bits or not?

And since we are talking about upscaling: Some anime grops are releasing y444 1080p encodes. ¿are they filtering/upscaling the
chroma for avoid aliasing on colored edges? or are they using 4k sources?

thanks