Martin,
Great report - we've all been there, faced with the dilemmas you describe. Conditions sounded pretty tough.
It's still a huge PB - even if you didn't hit the magic 1:30, so you've every right to feel pleased.
The problems with your HR underline why I tend to use mine only for rough guidance in a (longer) race situation. The elevated HR at the start must have thrown your calculations out right from the outset. But there's nothing wrong with feeling nervous - if I don't get that awful pit of the stomach feeling while I'm warming up, it invariably means I'm going to run a poor race.
I don't know how you felt during the race but from what you describe, I think your early tactics were sound. Living on the coast as I do, I've acquired a real hatred of running into a headwind, and if this situation arises in a race, I quite ruthlessly tailgate the runners around me (and I'm amazed how many of them just let me do it).
For me, your problems began when you let the group go - especially if you knew there would be another upwind stretch to come. To me, there's nothing more soul-destroying than being left on your own racing into the wind (and here, uphill as well!) - it's so easy to lose concentration and let your pace drop significantly.
Also bear in mind that, by and large, other things being equal, etc.... racing as part of a group usually has an effect on the HR - tends to bring it down a couple of bpm. How much company did you have over the second half of the race?
On a course where both wind and slopes were a major factor, it would have been really difficult to judge "a more even pace" - I don't think you need have any regrets about your pacing.
Sorry about the above - it looks like I'm pontificating. However, I'm sure I've got more experience of racing in windy conditions than most!
In 13 days I'll get a chance to practice what I pontificate in the Hoylake 10K. If the usual prevailing wind blows that day, the last 5K will be entirely upwind - and my pacing thereof will be anything but even.
Anyway, well done again - I'm sure with that time, if you can find a flatter, calmer 1/2M, you'll hit your target.