All good questions and observations. Probably laziness on the part of designers

I wouldn’t say laziness. More that the original Borg design would have looked terrible on the big screen.

Because (dun dun dun) visual continuity is and always has been flexible in Star Trek.

Probably for the same reason Gene gave about the Motion Picture Klingons: they always looked that way. But production was limited so we got what we got. (Very much paraphrasing. I remember reading that somewhere). It wasn't until DS9's Trials and Tribblations that his words were directly contradicted. Then Enterprise had to go and explain it.

As for the Borg, I believe Gene's words still apply. TNG season 2 didn't have the budget to give us the Borg they wanted, but First Contact did so we git to see what we should've had the whole time.

Which brings up an interesting question: if Gene were still alive, would he go all George Lucas on TOS and add the foreheads to the Klingons? Or the cinematic Borg to TNG?

* Edit:

It's already been established in canon that the borg and their ships looked different in TNG the best of both worlds (stardate 43989.1 (2366)) when they were less advanced, than they did in first contact (stardate 50893.5 (2373)) when they had evolved to be more advanced.