What to do if your pink slip has a stamp that says refused.

Ive been in the company for 1 year and 6 months and I wanted to resign but my employer refused so I went to LMRA for help which they advised me to send my resignation via post office and they gave me a mobility form.

Ive been informed by the post office that my letter has been return to sender since my boss refused to receive the parcel and now I got my pinkslip but theres a stamp from the post office that says refused.

I went to the LMRA and explain the whole situation they told me I already applied for mobility and just give the pink slip and resignation letter to my new sponsor and they can process it in their end.

My Dilemnas are:
1. Does it mean I can actually stop working on dec. 11 which is the last day i put on the resignation even if my employer didnt accept my resignation via post?
2. If my new sponsor tries to apply for me is their a way for my current sponsor to block it off and not allow the transfer?
3. My new sponsor is worried that my current sponsor will make the process longer and theyre afraid if it gets dragged on for too long they might back out from hiring me. Can my current sponsor legally make the process longer?
4. Can i still get my indemnity and my unused vacation leave?
5. Would i be tagged as runaway if i stopped showing up after the last day indicated on my resignation? Lmra said its fine to leave even if the employer refused but i just wanna be sure.

Thank you and sorry its a bit long but im just really in a dilemna right now. And i dont know what to do.