The last sentence works, but if you want to keep the "as well as" format, say: Are you saying that he was the person sending out the invitations as well as preparing all the meals?

It's OK except for the punctuation. You have a comma splice here (see e.g. http://en.wikipedia.org/wiki/Comma_splice ). The easiest way to fix it is to make two sentences.

No, it remains an adverb. The sentence is sort of a comma-splice error but without the comma, since it is a compound predicate nominative: He is the best in the business (and) not a novice.

Technically "preparing" should be replaced with "prepared" I agree. When thinking in terms of grammar, it seems wrong, and even when I read the sentence, it sounds wrong.

Short answer: no comma. "As well as" is being used as a conjunction, similiar to "and." The sentence could be reworded to replace the "as well as" phrase without changing the sense. For example: "John sent out

In sentences like this, I think it's as much a matter of personal stylistic taste as anything else. If you feel the commas help to make the sentence easier to read then include them, otherwise don't.

The commas are dutifully placed (except that you need one after 'So' in #5), but in fact all the sentences should be re-cast to avoid as many of those commas as possible.

The general rule on using a comma to replace omitted words is vague. You can do it if the words are implied, if the reader would understand them to have been said. It's a matter of practice. As for the closing example, I believe it is also

Could you tell me if it is OK to put/have a comma before a restrictive clause to allow/give a "breathing space"? I can't think of an example where a comma would be a good idea. The problem, it seems to me, is that a comma

I have two brothers, one is stupid, the other is very intelligent. This sentence appears to have three main clauses, which would undoubtedly render comma splices. But could you say there is just one main clause, with two dependent