<something> and <something> is enough or are enough?

I think the simple rules for numbering still apply here, so "x and y are enough" is correct.

"Will suffice" would also work in a more formal situation.

Definitely "is". The reason is because there is a singular subject, not a plural subject. What's the subject? The subject is reply. You must remember, the sentence really is :

My reply....is...

That should make it clearer. Thus, if your subject is singular, your verb needs to be singular as well. "is" is the singular form, while 'was" is the plural form. So use "is".

It depends. If you're saying "X and Y", then they are enough. If you're just saying "X. And then Y", it would be is enough.

Do the email and the reply cover it well enough? "are"

Or are you just mentioning the email, then saying that the reply covers it well enough? "is"