Usually, the ISBN number is supposed to change when the edition is modified.
The modifications may be minor and not matter much to readers.
The publisher is not supposed to change the text, except for typos and such,
though that could depend on contractual agreement in some countries. It may
also happen that some parts of the text have different versions, ...
Methods for estimating page count for ebooks depend on a variety of things and don't really indicate anything.
Different ebook distributors use different ISBNs. It used to be that print publishers would have a single ISBN. Nowadays, when you publish on a distributor's platform they will tend to assign an ISBN specific to the store.
Google Play stores ...
Re 3: "... the size of your cover image won’t affect the size of the delivered ebook…usually.
In some cases (usually due to an incorrectly formatted interior file), KDP will fail to correctly identify the cover, and will instead add a duplicate cover image. This is costly, as you’re padding the ebook with an unnecessary and unoptimized copy of your cover."