I'm sorry for that. Did you surrender your registration?
Yes, I did it.
You must report the theft of your plates and surrender the registration in order to obtain new plates and registration from the DMV.
If the plates were stolen from a trailer without liability insurance, would I have problems requesting new plates? Will I have problems with my license if fail to surrender a stolen plates?
Normally a trailer does not need liability insurance, and no, the DMV will not suspend your driver license if you fail to surrender the plates.
I'd also like to know if proof of ownership will be useful if I want to request for new plates?
Yes make sure you receive acceptable proofs of ownership
What happens if my certified letter is returned?
Bring it to the DMV office unopened.
Can the current owner then apply for duplicate registration?