Some practical advice: you may want to at least consult a competent lawyer who is both savvy about Japanese family law and the laws back home (stateside) before you sign anything or physically repatriate; such a person may be a
bengoshi or a
gaiben (
gaikoku jimubengoshi). In general, the laws of the former, aren't structured in the same fashion as US laws ... so it's fairly simple for a former spouse to renege on child support etc. If you require a referral I can do so (for what it's worth from someone on an anonymous board).
Also, don't take the physical security of you and your children for granted until you are safely back in the US. Don't wish to make you paranoid, but anything could happen including the grandparents snatching the children and the AH freezing accounts etc.
Also, if you require a sympathetic trained counseling ear in Japan:
http://www.telljp.com/index.php?/en/life_line/ (Tokyo English Life Line -TELL)
My sympathies to you and your children for what you are having to go through.