what are ways to increase the solubility of SnF2 at constant temperature and pressure.

I think adding a strong acid will work not sure what else.

Yes, adding HCl will do it because the H^+ combines with F to make HF and that is a weak acid.

Adding a salt that will ppt with F will work also. BaF2, MgF2, PbF2 are insoluble so adding MgCl2 should work.