TT bike/ Road Bike for 70.3

Great questions...as you noted this has been discussed a bunch but your details really frame the question better.

Three words stand out in your summary:  Never bike fit.  I think that's probably where I'd start, especially if you're considering a tri bike, where fit is much more important.  This is even MORE important as you move up in distance.  Those Canyons might not even fit you, even with adjustments. 

Assuming they do, though, you're talking about several thousands of dollars between options 2 and 3 vs. just not much between 1 and 2, but with a proper fitting you're going to be able to make a much more informed decision (and likely will be more comfortable on your Argon, too).

FWIW, I'm on road bike #2 after getting a road bike fitting six years ago post back surgery, and we mucked around with clip-on aerobars as well and I'm about as dialed in as you'll get on a clip-on/roadie setup.  I love this setup.  At 55 years old, I'm usually top 20% in the field on the bike for sprints/Olys.  I still get a little stiff once I get to like 45 minutes or an hour in the position.  I also have a tri bike that I need to get fit on - I knew based on my road bike fitting it would fit in general, but it needs some stuff moved around and lots of trainer time for me to adapt.

Interesting question. 

What are your goals for the HIM and all of this assumes that buying a TT/tri bike is within your budget for discretionary expenses. 

I've done 1 HIM (in 2006).  That was on an old Raleigh road bike with clip-on aerobars.  I did ride 20+mph.  I've done 3 IM distance races on an old tri-bike generally in the 18mph range.

Back pain/soreness after 36 km.  What was your training prior to that?  If you're sore after *every* 36km ride, it's probably an issue.  As you train, the early training 40km ride that leaves you sore is inconsequential later in training when you're routinely doing 70-80km. 

What is the HIM you're considering?  Is it a course layout that is really conducive to an aero setup?  There's no advantage to getting an aero setup and then not riding aero because it's a technical course, it has lots of hilly transitions, or you end up not being comfortable camped out in an aero position for he better part of 3 hours.  Riding aero puts a lot more stress on your neck since you're lower/flatter and still need to look out ahead the same distance. 

It's probably worth getting a bike fit on your current bike, either with or without clip-on aerobars.  As you ride a lot more, you might do another fit 1/3 through your training for the HIM (after you've started increasing to regular hour+ rides) so you have time to adapt to any changes in riding position before your high-volume riding (4-8 weeks before the race).  As you ride more, you can probably tolerate a more aggressive riding position than early on.