In my experience what can happen is a property owner gives a conventional first mtg, then gives a HELOC as a second. The owner/borrower then does a refi of the first with everyone expecting the HELOC to be paid off or the HELOC lender to agree to subordinate to the new mtg. Unfortunately someone forgets to obtain the sat or subordination agreement or record it.
The new mtg then goes into default and the lender seeks to foreclosure it. I can't recall working a title claim in Missouri but but some quick research found foreclosures are usually non judicial but can be judicial. In either case, since the HELOC lender is recorded prior to the mtg being foreclosed the lender is neither a necessary nor proper party to the foreclosure since it has priority of record.
In this kind of situation, where the third lender's money went to pay off the first lender's money I frequently used a theory called equitable subrogation to defeat the priority of the second lender. Unfortunately my research also found Missouri courts frown on its application. The Missouri Supreme Court in Ethridge apparently stated that for equitable subrogation to apply, the party against whom the doctrine would be applied (USAA in this case) must have "engaged in fraudulent conduct or committed acts bordering on fraud that created the condition that caused the lender a loss." 226 S.W.3d at 134. I doubt that happened here.
I'm surprised a new title insurer without existing liability is willing to accept a letter of indemnification from the old title insurer because as Tom wrote, a simple paydown of a HELOC doesn't mean it was satisfied, particularly since you advised USAA isn't willing to give a satisfaction . I'd be interested to know if the commitment and policy for the new purchaser will have an exception and affirmative coverage for the USAA mtg so the proposed insured will at least be aware and hopefully understand the very real possibility of a foreclosure action being brought by USAA. If the buyer is getting a new mtg I can almost guarantee the commitment and policy don't mention the prior mtg because most lenders sell the mtgs they originate and most can't be sold if the Mortgagee's Title policy has an exception for prior liens, even if there is affirmative coverage over the defect.
Title insurance companies can be all to willing to issue a policy without exception for a defect assuming the prior title company will take care of the situation if it ripens into a claim. Unfortunately if USAA files foreclosure it may be sometime before the issue is resolved during which the insured will probably not be able to sell or refinance and will have to worry about possibly losing the property.
I believe you wrote the HELOC was for about $60k. Lets say you're selling the property for $150k and your buyer gets an owner's title policy for that amount and to make things more interesting let's say the buyer gets a $150k VA mtg. USAA files foreclosure and can prove it's owed $160k with accrued interest and attorney's fees. The title insurer might decided its cheaper to pay the loss to the insureds rather than fight USAA and incur say $20k in attorney's fees and then lose and have to pay the policy amount anyway. In that case the VA lender would get paid the amount its owed on its mtg, probably $150k and the buy/borrower would get whatever is left under the policy, probably zero.
In addition, you might want to talk to your attorney, one who represents you about your obligations to disclose the mtg, particularly if you're giving a Warranty Deed.