If the client is setup as the Administrator on the PBGC site, they can grant you access by inviting you. If the prior TPA did everything you might need their assistance.
As for the premium, if it's a small plan using the lookback, the premium might be high since the interest rates lag what's been going on.
Again, just had a similar experience. I ended up drafting aa detailed email outlining the professional responsibility of the prior actuary to transition the work. That did it. Not the best nor the enjoyable experience but it might do the job.
If you get the access to the PBGC MYPAA portal you should be able to see the history of prior filings right there. Which might be a challenge if the prior TPA is the "most non-cooperative". I just had the similar experience - the prior TPA made sure everything is the most difficult for the takeover.
Stop there. Refer him to the SPD, which should in turn refer to, or include, written QDRO procedures and claims procedures. He is making a claim for benefits. Make him follow the claims procedures. Stop trying to explain or coach.
Assuming the plan doesn't currently allow for discretionary match I would assume they would need to adopt the amendment before they started depositing the matching contributions to the plan but not later than the end of the plan year for the discretionary amendment.
That said, if they did start depositing the match early, I think the eventual amendment would most likely work as a self correction under the newly expanded IRS rules.
With Appreciation, "trust agreement" normally means the legal document between the employer(s) and trustee(s) creating trust. The distribution and other disbursement ledger is usually called a "trust report." Just sayin'.