The good thing is that if there is paint left over in spots after it is stripped, it really will not matter because your painting it black anyway.
I wouldnt use shellac. If you sand the piece some when you get it back or not, wipe it down good with naptha and blow it off either way first before applying anything to remove oils if any.
I would use a sealer, not shellac. I would apply 3-4 good coats of a high solids sealer, sanding each coat in between. The first coat of sealer I would scuff sand with 320 grit, then the other 2-3 coats I would sand with 240. Then I would apply a coat of black, scuff sand with 320, then apply another coat of black, scuff sand with 320 and apply 2 coats of acrylic lacquer and that will give you extra depth. Its sounds like alot of work, but you will be pleased with the end result.
If your going to apply silver leaf, of course do that after the 2nd coat of black and apply 2 coats of acrylic lacquer.
I just finished an oak piece where the grain had to be filled in completely. Of course we were out of pre fillers so I had to apply 4 coats of sealer to get a complete grain fill, so I know you should be ok with the above.