Edited in response to moderation (possibly brought on by flagging my own post):
Yes.
For example, there are upper limits on the size of insects due to (i) thickness of exoskeletons and (ii) transporting oxygen via trachea.
No. They usually point to the most effective shape for competing in specific environments, such as (i) streamlined shapes of tuna, sharks, dolphins and ichthyosaurs, or (ii) sabre-toothed predators on very large herbivores.
The same way we think about any other explanation: does it make testable predictions that match later observations.
More broadly:
Completely, since standard evolutionary theory includes constraint-based explanations.
You not only posted a link to an earlier version of the same preprint 18 months ago, but even claimed some responsibility for producing it.
So presenting it as something you “recently came across” is not remotely credible, and undermines anything else you say about it.
This thread is a sham that should probably be closed.