MYP or DP?

You can start by reading the guides for each subject, which you can usually find on google if you don't have an IB logon yet. But surely they will be sending you to training right?

If it weren't for the 6 preps, I would go with getting IB experience. But 6 preps plus less savings to pay off loans sounds like a lot of stress for a line on your resume that, at the end of the day, could mean very little. If you work on implementing strong curricula at IS 1, that could be just as impressive when you search for your next job. I could be wrong on this, but I also would think a future recruiter would realize you couldn't possibly have done a good job with 6 preps, and maybe question what that prior experience could really bring to their school.

I've had a similar experience and think it is true that it is easier to get hired internationally. Just think about how many hoops you had to jump through for applying to domestic schools- usually multiple in person interviews, essay style questions submitted, sometimes a video or demo lesson plan submitted, and sometimes even taking a day to fly to the school and teach a lesson/ interview with staff. You can get an international offer after one 45 minute Skype interview. To me there is no question.

I think maybe what you've heard about "bad form" relates to attaching lots of individual documents. I would put everything into one pdf file and the body of the email can be a short blurb of introduction, explaining you have attached the materials.

I was recently in a similar situation- I had accepted an offer in writing, and the school had even sent me a contract. I was surprised to find out that this wasn't actually enough, and a few days after sending out the contract, I found out that the position may or may not exist. I hope things turn out better for you!