Brian2828: get out your table or dive computer, plan all the dives and see where your residual nitrogen is going to be at the end. Look at your final pressure group.
Unfortunately, the dive plan outline appears to be well beyond what normal (Recreational) tables - - even some of the older ones - - will provide guidance for.
For example, 18hrs BT in 3 days is 6hrs/day of bottom time, and as per one old table that I happen to have handy (old PADI, IIRC, copy of NAVY), the highest bottom time value on the table for 30fsw is 310 minutes = 5.16 hours (FYI, this returns a Group "L"). Similarly, the DCIEM model (1992) has a max no-stop bottom time of 300 minutes @ 30ft, and Bassett says 220 minutes max. And surely, while any diver would have some RN Group crediting from SI's, the question isn't
if he will go "Off Table", but merely
when that will occur, as per each of these example Tables' models.
Even here, it doesn't necessarily mean that the diver's at risk - - just that he's beyond the point where the model has had sufficient
validation with which to have published specific NDL (or Deco) guidelines.
When on vacation, I do not dive on the last day.
Unfortunately, there's so many variables in dive profiles that even DAN (who did suggest a 24 hour SI some years ago) has revised their current guidance of 12 / 18 hours being dependant on insight on just what kind of dive profiles were being performed.
FWIW, a far more common 'dive travel' scenario when assessing the comparative risk question for flying-after-diving is if one should perform a short/deep "bounce" dive profile or a long, shallow ("take it easy on my last dive day") profile: the answer resides in the details of the deco theory's math, which may very well be non-intuitive.
Unfortunately, given the current scope of mainstream recreational dive education on deco theory, I'm afraid that the OP's question is simply out of its league, and similarly, some (if not most?) of the deco models being employed today are also inadequate because many models simply didn't undergo sufficient validatation.
The simplest answer is to suggest "Don't Dive This Profile", of course, but that's already obvious and known, and it does not really serve to answer the underlying question of the objective assessment of the potential risks. Of course, this is multivariate and there's other risk levers that can be pulled, so if we were to look at the question more holisticaly, the two things that IMO would probably be more significant than the 18 hour (vs +X hours longer) SI to flight home question is if we've adequately managed the risks of: (1) the diver remaining sufficiently rested & hydrated during these three days, and (2) if the diver has been specifically tested and confirmed to not have a PFO.
-hh
