Do I keep the raid pass after failing?

Nope, you are not able to keep the pass, it is that after you use the pass you can enter the same raid as many times as you want until the time of that raid expires or you succesfully defeat it.

But the pass is already "consumed" on the first attempt, just before you enter the lobby. So if no ones shows or you fail once the time for that raid ends you will have no pass.

If you enter a raid, you use the pass. If you fail to defeat it, you can keep battling it and re-enter the raid till its timer is up. You're not necessarily using the pass twice because you already used it. The raid is just open to you till you defeat it.

You can go away and come back to the same gym using the same pass.

I was trying a solo level 3 vap, close but getting beat. Ran to a raid (different pass) that just started down the road. Beat that one, found someone to come back and help beat the vap (same orginal pass).