'framer' lots of good info being postulated here:-) It's great that you started this thread as we can all learn or at least be refreshed but I think perhaps yr over thinking this a little with regards what's legal & where it say's as much.
Look at the end of the day you need to land with yr fixed fuel res intact (preferably with variable also) To simply put this whole thing in perspective you work back from the above. At any stage of the flight you need to land with the appropriate fuel on-board whether it be at yr original destination or any Alt you had planed at the planing stage (if it still applies wx wise), that's the legal part of all this & as has been said B4 how you go about that is up to you & on the day.
The reg's are laid out in Jepps for basic pre flight plan fuel req's. Flt fuel encompassing all flight phases (inc any Alt), fixed res, variable res, holding (whether that be for wx, traffic or any other contingencies such as airframe issues where a performance penalty has to be applied etc) & of course extra fuel for being a commander with doubt (about the wx for Eg).
We all know the above or where to find it inc in Ops manuals so a good read of the Jepps for Eg will answer pretty much all yr q's. All the other reg's CAO's for Eg aren't usually referred to in day to day operations otherwise we'd all be still in the briefing room trying to understand the at times convoluted rules in which we commit aviation under
I refer back to the recent MIA incident again. Those guys took off am sure with the req'd fuel but got caught out as we don't live/fly in a perfect regulated world as per the Reg's.
We have Reg's that will keep us as safe as possible if we stick to them 100% but no Reg's will ever cater for the unexpected, there in lies the difference between Commanders whom not only know the Reg's but know the environment we all fly in & make allowances for that
.
Wmk2