Other than acetone Im unaware of a solvent, not sure about getting thru the wood to soften. Ive also not seen where some one reglued a large expanse of existing 'rubber' roof. Sounds problematic at best, not to say it couldn't be done. In conjunction with other roof issues youve had, I might be considering stripping it, laying out new luan on top then reglueing a new roof.
Other consideration is cutting it back few inches to good, replacing rear few feet, If there are any rv shopes near you you should be able to buy epdm by foot. Cover the seam with 6" eterna bond tape.
I dont see an easy way.