Acetone and xylene are the accepted solvents for balsam. But is your lens cemented with balsam or epoxy/UV curing adhesive? You need to find this out and use the correct method of decementing the lens. Someone posted here that boiling the lens in water would free the (non-balsam) cement.
As for damaging the coatings, acetone shouldn't harm them nor should xylene nor boiling.
Must not have been porcelain, porcelain is a type of glass and acetone will not dissolve it under any circumstances.

The best and brightest idea I have is to learn the ins and outs of lens recementing on something a lot less valuable than a Planar off a Hassy, unless you feel able to buy the owner a new one. Learn on some old R-R types.