It is also pretty clear that valve will not help Epic make fornite more compatible on their platform, as they are a direct competitor.

Wrong, Proton is open source and Valve would still benefit if Fortnite succeeded on Linux as it'd grow the ecosystem they're investenting in. Valve has said themselves they're open to supporting any game that takes advantage of Proton, including competitors. Unlike Epic, they're not trying to monopolize the entire market. If they were, they'd be trying to make deals with Microsoft to come pre-installed or some other invasive shit like that.
Hell, Valve already dead ass worked directly with Epic Games to add Proton support to EAC & EAC support in Proton(proton_eac_runtime) in the first place. Why the hell wouldn't Valve be obligated to support them?

because when shit doesn't work, the blame goes to them and not the layer. And that's bad PR.

All they have to do is say "running under Valve Proton report bugs here↗" similar to what Steam does, problem solved.
Not to mention, Linux users are 1000× better at making actually useful bug reports.