As far as next plans go, I've decided to cut out the top section of the helmet, and re-pep it. it's too warped at this point to be viable.
Honestly... I wouldn't bother. You're going to have a very, very difficult time matching the new work to the old, and even so, there are a dozen smaller issues aside compounding your problems. I'd urge you to try a smaller, less intricate piece at first, to practise on - perhaps look at working on a Mk VI shin piece.
Don't make things unnecessarily hard on yourself. Treat this as a first attempt, learn from your mistakes, and re-do the Pepakura work from scratch. Take it steadily the second time around and make absolutely certain before you resin that your shape is good and rigid. Following that, try to keep these little hints in mind when you're working with your chemicals:
- Less is More. It's far easier to layer up than sand down, and you don't have to fight against your product's cure time to get it all on the helmet before it sets. It's also a lot cheaper since you're not wasting so much product on sanding away tremendous amounts of excess.
- Speaking of which, be mindful of your product's work time - if you're finding that you have to rush to get your bondo on, try using less hardener to give yourself a little more work time.
- When applying your bondo, make sure you're using a decent, straight-edged spatula. Imperfections in the edge of your applicator will cause grooves and bubbles in your bondo that will consume a lot of time and effort sanding out and filling. Personally speaking, I use an old plastic loyalty card, but you could always use something else like a paint stripper.
As for answering your other questions:
- If you're having trouble getting into tight corners, consider using a smaller applicator - I've used the cut-off end of a popsicle stick to great effect in smaller areas. Just take a small dob of bondo and work it GENTLY in until you're happy with it.