> Are the Old Bohairic and Greco-Bohairic pronunciations mutually intelligible?
Yes and no.The spelling is exactly the same so in written form they are identical.
The pronunciation is different but it follows very specific rules, if you know it you could understand the other person.
for example to go from GB to OB you need to make these changes (v -> w, av -> aw, th -> t, y -> ae (sometimes), ....) the other thing is that GB tend to break a word into more syllables. If you are aware of these differences, you could understand otherwise you can not.
> Were the way Bohairic and Sahidic were historically pronounced mutually intelligible?
Yes! Even spelling differs greatly and certain words only exist in one dialect but not the other. A person from south Egypt would need a translator to communicate with a person from the north and vise versa. Although of course being dialects of same language, it's very easy to learn one if you already know the other