[Kansas58]

I have it on trusted authority that you are at least partially correct.
Yes, they do manage the accounts, but that should not matter.
SIPC insurance would take care of any making sure that investments in the account were “delivered” to any new firm.

[Orange1998]

"PBGC does not index benefit payments. However, by law it must increase the maximum guarantee limit each year to reflect the increase in national wages. For plans terminating in 2004, the limit is $3,698.86 per month or $44,386.32 per year for a single-life annuity beginning at age 65".