Self Seeding and Cutoff Times!

Not sure which race you're doing, but consult the athlete's guide and if you want, contact the race director about how the start and cutoffs are implemented.  

Agree, with a 1:50 swim and 8:00 bike, you've got 40 minutes available between a 7:00AM start and a 5:30PM run start cutoff (which you should confirm).  10 minute transitions aren't unreasonable, but puts you in a 20 minute window to actually start swimming.  In the past with a mass start, that is plenty.  In time-trial starts, they can usually get 12-20 people per minute (3-5 second intervals).  

Obviously, given your concern with potentially missing the 5:30 cutoff, you wouldn't want to seed yourself any further back in the swim than necessary.  (Expect a few others to have the same concerns.)  Some slow swimmers will blast the bike, and you might plead to start ahead of them (hey...they get to pass you on the bike, so win for them...)

I ran across this article that may be helpful to you - it talks about how to decide where in your allocated slot to position yourself based on what type of swimmer you are (elite/good/poor). I personally liked the tip about positioning yourself for the most relaxed approach... if you have swim anxiety, this may mean starting towards the back of your wave is actually more advantageous. That kind of thing.

My other piece of advice would be to stay focused during the race - which can be hard to do when you're out there for 15, 16, 17 hours. Continuously be thinking about your next move, next mile, next nutrition checkpoint, etc., instead of zoning out or daydreaming (guilty here!). As you near the end of the swim, visualize yourself coming out of the water, running (at least jog! Lots of time is lost walking in transition) to your bag, getting what you need, what order will you do things in transition, jogging out of transition and smoothly mounting your bike. Do the same as you get ready to finish biking. For me, this helped eliminate some of the fumbling around time where you end up saying "what am I doing?!" and spend valuable time re-grouping.

Last, if you haven't already, check out race reports from folks who are around your speed (I may be one of them ). Read up on tactics that worked and didn't work for them, note what they wish they would have done differently. Best of luck!