Main problem I see is even if he continues his torrid May pace through to the deadline his April was so atrocious his overall line will still be middling by the trade deadline. In 2021 the Braves got Rosario with a typical Rosario stat line at the time for a washed up Pablo Sandoval. I don't see how the Nats will be able to flip him for anything of value.
He can help the Nats win games in the meantime though I suppose.
I'm not sure whether smart teams look at whole season numbers at the trade deadline or break down numbers into smaller bunches when they become stabilize. Some of the plate discipline numbers get to be stable in less than 100 PAs.
“Stabilization” Points for Offense Statistics:
60 PA: Strikeout rate
120 PA: Walk rate
240 PA: HBP rate
290 PA: Single rate
1610 PA: XBH rate
170 PA: HR rate
910 AB: AVG
460 PA: OBP
320 AB: SLG
160 AB: ISO
80 BIP: GB rate
80 BIP: FB rate
600 BIP: LD rate
50 FBs: HR per FB
820 BIP: BABIP
https://library.fangraphs.com/principles/sample-size/For Rosario, walk, strikeout, and HR rates should be stable by the trade deadline without the April poison. Maybe groundball and flyball rate. I think he'll be evaluated on the talent he's displaying. Scwharber's bad April wasn't much at issue at the deadline in 2021.
I agree with your second point - he can help the Nats win games in the meantime if April was just "he always has a cold April, and he had no spring training."