First, let me state that I am still in the process of packaging my application so I'm pretty fresh myself but in my opinion, the starting point in applying for hsmp is possessing the right attitude/frame of mind. Do not be scared; refocus your mind on evaluating the criteria and seeing how many points you can justifiably claim. Go through the posts in this forum and you'll find much assistance already existing on the forum. All you need are 65 points and evidence that you can continue your career. That latter part about continuing your career is a walk over for you - there's huge demand for teachers, I hear!
Professions/careers defer but as far as I know, hsmp is not career specific. It therefore means that within the broad guidelines given for 'senior level positions', activities undertaken will defer.
Since your post was majorly about claiming senior level position, let's consider it:
Going by the over-28 guidelines, all you need are 5 years graduate level work experience with 2 years senior level included in the 5 years.
Head of Maths Dept for 3 years: I expect that to count. If the period during which you wrote the course curriculum falls into that period, the senior claim would be augmented.
Wrote the curriculum for 2 of the courses taught in your district: I would give a shot at claiming points for achievement. That should qualify under published work, I think! That would be 2 pieces of evidence for achievement. You need one more - read the guidelines carefully. Did you receive an acknowledgment from the Education Authority? If no other evidence at hand right now, you could consider getting peer reference as your 3rd evidence.
Having said all that, from what I've gathered, rules for senior and achievement points are very stringent (but aren't all the rules?!). Come on, give it a shot. Claim all the points you possibly can - reasonably!
Wrt resigning your post, I believe the general practise is to give notice of resignation. I'd suggest you wait until you receive hsmp approval before putting in your notice. Unless you've contracted to give very long notice, I guess you could take 1 or 2 months to sort that out before relocating - you'd have to process EC anyway
Wow, what a long post! Hope it helps.