These are very good questions. Your logic was spot on in creating (2a). All four of your examples are grammatically correct, and your (a)s are essentially synonymous to your (b)s.
Usually, adverbs that modify adjectives occur before the adjective they modify. "Enough", when used in this sense, tends to be a rare exception.
Also, adjectives usually precede the nouns they modify, as with your (b) examples. In the (a)s, I would say that the postpositioned adjective phrase is a reduction of an adjectival clause to the effect of "that is ... enough":
1a': He is a soldier that is brave enough to attempt it.
2a': I don't have a jacket that is big enough.
The reduction (the elimination of the words "that is" that identify the phrase as a clause) is very frequently used, and, as far as I know, universally accepted.
I hope you find this helpful.