I’d ask to see this metal that’s in the oil. Did they even bother to test oil pressure? Check for vvt codes? You probably have sucker written across your forehead.
With you stating that you noticed no top end noise after driving it on the highway for 10 minutes, i’m thinking that it has oil pressure.
The oil pump on that motor is driven by the snout of the crankshaft. I just can’t imagine it working one minute, then not working the next. Growing weaker over time maybe, with the oil light flickering at idle as the first signs is a typical failure scenario. The only “now it works, now it doesn’t “ scenario I can think of would be the pressure relief valve on the pump is stuck, which I believe is accessible from outside the timing cover.
Those 3.5L motors clatter like hell after an oil change until the filter fills up, so a no pressure issue would seem obvious to me.
You can remove the oil fill cap and look in there and see rocker arms and watch for oil flying around as it’s running too.
Unless it has obvious vvt noise and vvt codes, I’d have the pressure actually tested before i went any farther. If they don’t have the means to test the pressure, have them replace the $30 pressure sending just above the oil filter and drive the fucking thing.