Optic gurus here? Questioning effectiveness of OA on parallax in compact scopes

[Relentless Holiday]

Maybe we got an optometrists or some NASA telescope genius who is around for stuff like this. I have a question for someone who really has the lowdown on how light is refracted through the lens sets in scopes... Three question actually.

I am wondering if there is a reduction in ability to reduce parallax correction as the tube length, and subsequent shorter eye relief, become factors in really compact scopes like the UTG bugbustres and even cheaper Sniper brand compact scopes?

I have noticed recently that apparent parallax movement that seems absent in longer scopes is pretty plainly visible in my shroties.  I can brace the shorter scope and with pretty minor changes to eye position I can move the POA considerably.  Longer scopes seem much more forgiving on cheek weld.

Also, will adjusting the eye reticle focus ring, to get a focused reticle, affect the parallax adjustment effectiveness?  I recently tore a retina and needed an emergency laser surgery.   It left my right eye a bit more impaired so even tho I tried not to use the focus ring thinking it may, but I might need to in the future.

I guess if I do find a scientist type, I would also like to know if the measured parallax distance shown on the scope should be used, when the true target distance is certain, after making big focus adjustments at the eye piece?  Or would parallax be corrected to the max when the marked distances are ignored and target focus is prioritized?   I noticed it changes the target focus when the eye focus is adjusted

I would imagine that the math of this stuff is dependent on the actual configuration of the assembly.  But I am inquiring about typical second plain of focus retical scopes...if that matters... specifically the two mentioned above. 

Thanks for any info.