In regards to United, I believe the pensions for all affected employees, regardless of job classification, are in the lap of the PBGC. Hard to tell for sure with all the complex wrangling going on at the present time.
If the CSPF were to default, from what I understand the PBGC would step in. An excerpt from the GAO's "Report to Congressional Requesters," on private pensions from March 2004, titled "Multiemployer Plans Face Short- and Long-Term Challenges" (GAO-04-423) states on pages 17 and 18:
"Agency officials told us that troubled plans often solicit their technical assistance since under the multiemployer framework, affected parties have a vested interest in a plan's survival. Occaisionally, PBGC is asked to serve as a facilitator where the agency works with all the parties associated with the troubled plan to improve its financial status. Examples of such assistance by PBGC include facilitating the merger of troubled plans into one stronger plan and the "orderly shutdown" of plans, allowing the affected employers to continue to operate and pay benefits until all liabilities are paid."
Furthermore, the "PBGC does not take over the administration of multiemployer plans, but instead, upon application, provides financial assistance in the form of loans when plans become insolvent and are unable to pay benefits at PBGC-guaranteed levels."
Having said all of the above however, I wouldn't be surprised if there are other multiple layers of complexity involved.