Actually, I have done the above quite a lot. I have a very detailed offline family tree that I continue to garner sources into. If I have profiles on WikiTree that I feel could use a bit of freshening up with added sources from my offline tree, I have no qualms with uploading a new GEDCOM. The process of bringing in individual profiles is a bit daunting with some of my larger lists, but updating profiles on WikiTree from my uploaded GEDCOMs has really been pretty easy.
When I import a GEDCOM that has profiles for people I have already imported, but has added information, I go ahead and compare the two side-by-side and copy in whatever changed biography information I have.
When you compare them, you get the option to edit the match through your GEDCOM import, with a button on the GEDCOM side that says "EDIT Name-WikiID" of the profile on the right. Click that and you'll get a merge page which gives you the option of selecting which information to keep. At the bottom, you can clean up the merged biography, and include only the information from your imported GEDCOM or keep the original information. It is always important, however, to clean up your biography, and ensure that it doesn't have multiple GEDCOM imports stacked up.
As to your number two, I'm not entirely sure which branches you're referring to; I've always found it much easier to find connections to the broader tree by importing various branches through GEDCOMs and seeing what profiles already exist and matching those with them. My connections are quite thorough, from this method.
I hope that you find the genealogy method that works best for you, whether it's here or elsewhere. Thanks for being a part of our community!