the logic is: if a=true, then b=true
非 is used to indicate condition a = false, 不 is to show that condition b = not true.
so, rūpa, (色, one of pañca-skandha 五蕴) appears (生),
by this clinging (生), hence (故) one think (計) "i am" occurs (是我).
非 - if the condition is false --> rūpa doesn't appears
then 不生 (one would not think that "i am" occurs).
it's my best try, the buddhism terms might not be proper.
an other example:
for example (譬如)
a man (士夫) helds (手執) a mirror (明鏡),
he sees 自見 his own face (面) appeared (生) in the mirror,
the clinging of a mirror (生) causes (故) the seeing (見) of one's face.
非 : if no mirror in hand,
then, one cannot see his own face appeared (不生)
imo, "不" is not "non" (such as "non-stick pan") in this sentence.
based on reading experience: "buddhism text" is closely related to literary chinese, but, it has its own styles, grammars, and developments. you used "hybrid", middle (?) to decorate it, very nice try :)
"生法計是我﹒非不生" is "upādāya asm-ti hoti no anupādāya"