To rewind the film in mid-cocking, I put the lens cap on and fire the shutter. It shouldn't stress the shutter too much. Later - as usual: rotate the collar, rewind. No need to waste the film this way or another. If you don't have a "test" roll, I think it's easy to make one, some film and an old canister is all one needs to check things or practice.

I don't quite understand the part about overexposure. Is only 1/30 time affected? Or all or all the slow or fast speeds? Constant 1,5 stops off is almost unimaginable with this type of shutter, but if it's 1/30 only and it doesn't look like "lazy shutter", my guess would be the flash issue we've been talking about earlier has indeed something to do here. Lazy shutter curtain would also affect the slower speeds, at least you should be able to see it spoiling 1/15 also.