I don't disagree with you, but the main points:

1. You are a hardened IM athlete training 15 hrs a week for a long time. OP is a 70.3 athlete, with presumably no or remote IM experience. BIG difference between IM volume commitment and 70.3. Sure, you CAN train as hard and harder for a 70.3 than an IM if you so choose to, but I'll wager the vast majority of AGers who aren't national class train a lot more for the IM.

2. It's hard to add enough run conditioning in <2 months to go from a max long run of 13 miles to 20 miles without a high risk of injury or burning out your legs for race day, even if you are 70.3 race-trained when starting the transition. At least for IM, you're doing a few runs over 14 miles, if not more than 18 miles; in 70.3, you never do that run, and you might even max your run distance out at 10-11 miles as in a lot of plans. That's a lonnnnng way to go for 26.2.

3. I will wager you are a lot more talented than you think. To compare, I will compare myself to you, where I have done several blocks of nothing but pure run training of 70+mpw with quality (Pfitzinger's Advanced Marathoning 70mpw and up plans), and never gone sub 3:10, even in my early 30s. There is no way I can run faster than my peak back then, and I still could not run as fast as you. And I'm no super-lame ability person either; I walked into those Pfitz plans running a mediocre but not-slow 20:xx 5k, which is already faster than most AG triathletes can run standalone 5ks. (Got that down to low18s with all that mileage.) I also ran a flat HM after a strong training block 70.3 for myself where I couldn't go faster than 1:34, and I know for sure that time would have slowed catastrophically had I attempted a 26.2. So I suspect you actually are one of the people that runs a lot better than the average AG triathlete on less miles trained. (Still give you props for your 15hrs/wk!) I'm going to guess you're one of the fastest in your AG even at big races and are USAT all-american?

From my view point I think that most who have replied are correct. But it also depends on the goals of the OP. I have ran marathons with 70.3 and IM training and have done well (2:50-3 hours) but once I tried to get to the low 2:40s I found that I could not run that pace for 26.2 miles with only 4-6 weeks of marathon specific training. At that point, for me, I needed a lot more miles on my legs to be able to run that pace for that long. I do agree that being properly trained my 70.3 pace has been fairly spot on to my marathon times. But seriously, whoever said that the marathon is a different beast is 100% correct. I have never been more sore than after a marathon. Also the mental games, to me, are harder than rough patches in an IM because it is ok to walk in an IM but it is NOT ok to walk in a marathon (well it is but no where near as many walk in an open marathon)!

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So maybe this is just me but I see little difference in total training volume between a 70.3 and a 140.6 (except for maybe the last couple of weeks for the full IM). The difference is in the type of volume. More faster pace/higher power work for the 70.3; more emphasis on longer endurance efforts for 140.6. When training for a 70.3 A-race my long runs usually go out to 15-16 miles, usually with tempo intervals in them. When training for a 140.6 I might not go past 20 miles on any one long run, and in a few builds have topped out at 18. So depending on how the OP trained for that 70.3 the long run differential may or may not be a hard gap to bridge. If, as you say, his long runs topped out at 10-11 miles, then maybe that would be a stretch, but not an insurmountable one depending on how much time there is between the 70.3 and the marathon.

This topic comes up in various forms fairly regularly. I think the latest iteration I saw was "Is a :49 in the 100 freestyle fast."

There are always people who have a stricter view of what constitutes exceptional and those views typically comes from inside the sport. I suppose the reason for that is a coach or athlete will encounter someone with no work ethic but supreme talent who achieves a :49 (or 16:30 5k) AND will also encounter someone who has "mediocre" talent but great work ethic who also achieves the standard. They will then conflate what they have seen and conclude, hey, anyone can do it. I still maintain that a :49 is exceptional to the degree that it is something only thousands do out of hundreds of thousands. But I can see the other side of that argument.

But a 16:30 5k... come on. That is exceptional. The BIG difference between running and swimming is that, for the most part, anyone who CAN run a 16:30 does it. Every kid runs, the good ones get noticed and end up being...runners. You give me a 100 freshman boys who can swim one lap of freestyle competently but otherwise have no competitive swim background. You give me 4 years to work with them, I bet I can squeeze a handful of :49 100 freestylers out of the group. But if we tried that to produce 16:30 5k runners out of that group you might not get any.

With a goal of 3:30-3:45 and your run pace during HIM's, you should be fine assuming you get in your long runs after the 70.3. We have similar HIM run paces and I went 3:48 when my open half mary PR was 1:45. My weekly volume at the time: Rest-5mi-8mi-Soccer-Rest-Recover from hangover-Long Run. I was also in my early 20's, so my recovery times were pretty quick and I was much more durable then.

Assuming you only have about 6 weeks between races, I would make sure you hit a 15 miler before the 70.3. Then after the 70.3, you could have long runs of 15, 16, 18, and 20 before a taper.

Good luck and have fun.