I suspect we may be being penny foolish here.
No we don't get the FVF refund on partial refunds. Which is not fair.
But if your customer paid $10 shipping the FVF would be $0.90 and if you refunded $5 shipping the potential refund to you would be $0.45.
For the very occasional refund, not enough to worry about. (Still fattens Paypal's coffers, though.)
And if you are right about the doubled $0.30 fee, then you would be out $0.15 by trying to get the partial FVF refund.
Plus your time at $0.17 a minute minimum wage.
Still fattens PP's coffers. Grump.