The second part of the proposed change is NAD; the rule is correct as written and adding "is" would not be grammatical.

All of the conditions given here are "positive"; the bulleted list is describing the prerequisites for performing this transformation.

As a Chinese, I'm not familiar with this type of sentence structure either. I have no clue how the phrase "were the allocation not extended" can be attached after an "if" sub-clause...

The implementation may extend the allocation of a new-expression e1
to provide storage for a new-expression e2 if the following would
be true were the allocation not extended:

... is a more concise (and, to me, slightly more natural) way of writing this:

The implementation may extend the allocation of a new-expression e1
to provide storage for a new-expression e2 if the following would
be true if the allocation were not extended:

"were" is introducing a counterfactual conditional, whose meaning is this:

The implementation may extend the allocation of a new-expression e1
to provide storage for a new-expression e2 if the following are
true in a hypothetical world where the allocation is not extended:

... except that the latter phrasing seems even more unnatural because 'would be' and 'were' are more appropriate forms for talking about hypotheticals, rather than 'are' and 'is'.

If we can express this sentiment in a way that is both natural and avoids this complex grammatical construction, I'm happy to change it.

I got it. Yet another nice use of subjunctive mood... Thank you for explaining it.