I don't really understand what you are saying when you say bottle after two months and sweeten with wine stabilizer....
Can only suggest time will lessen tart. I have a gooseberry that was absolutely astringent right from day one. Just stripped all moisture from tongue, mouth, throat all the way down...horrible crap. Left it for months between racking to put off what I thought would be a dump job. After 18 months it's still a bit drying in the mouth but has a choice aroma. Will give it a bit longer and if it doesn't tone down any more will backsweeten with stevia to remove the sharpness.
To go back to your original post I have seen postings, I think on this forum, regarding boiling or heating to nullify preservatives. I have boiled, simmered for a period, and have had success. So I say worth trying.