Why do you want the head separate? Its a very commen problem caused by looking ate the needed end result too carefully (the real asver lies outside the easily seen answers, thus can never be seen by focusing ine the problem). Theres 2 bad reasons as fara s i see:
[ul][li]you want to have a separate head so you can animate it without needing the full body.
Mental problem: the 2 things are separate issues. Nothing says that having the head not separate precludes you form doing this. In fact the srather supprising thing is that you get a lighter model if yoru head is really part of your model. The separate head is very easy to make out of the singke piece model just delete all faces you dont need run the blendhspae trough all its poses copy delete hiostory and viola you ahev a identical bklenshape thats both LIGHTER then trying to merge it with teh rst of the body.
Bonus it now works with subduivsion surfaces.[/li]
[li]You think yoru going to save time. Well really thatsa very big IF. See its true its easeir to model separate heads, as its lighter.
Mental prblem: you assume that building a full face body rig is hard to do form the severed heads. It is not its simply a 10 minute one time investement script to automate. Its also superor to having a separate head since ist lighetr on yoru model since you remove.
this is basically the reverse of problem number 1 where you jsut blend the sevred head attach it and merge edges to the body. However please dont leave it there, since you now have a 5 times more slower model than it needs to be. The combine and merge operations are aquote heavy for your computer. They can be removed by again extractinga ll the compound blends and remerging them into one belnd aftera history collapse. (this is very easy to automate)
if your smart you save the heads in one file make the body merging script then you can jut copy over the blenshape node to yoru main rig file thisway you can add blends easily after the fact.[/li]
[/ul]
wheightpaint out the influence of the body?
Why would you need to paint weights? The belndshape actually does discard all the points that dont account for moving* so the comoputer allready automatically optimizes thius out for you.
How to make two seperate polygon object to look like they match?
Well to be hones you can do this but you dont really want to in this instance so theres really no point in going trough that.
*in fact it does this BETTER than you can hope to acoomplish by hand as it does this per blendshape. Wich yu have no hope to accomplish.