Ill tell you from experience what I found out directly from an FAA inspector
Congested areas are considered any dwelling or open air assembly of persons. Contrary to popular belief it is not just the yellow areas on the sectional. For example if two people are walking in the middle of an open field and you do acro over their heads its a bust. Acro over a single home is also not legal. In any even if you piss off the FAA bad enough theyll bust you on 91.13 and they wont need to worry about deciphering the language of 91.303
As is always the case with the FAA interpretation will vary by FSDO and inspector. If you call your local FSDO and ask to speak to an inspector he or she may clarify this for you and even put it in writing so you can back yourself up in case something happens.
With respect to your question on Jet airways. They start at FL180 so unless you practice acro REAL high I dont think it will be a problem for you.