Yes. Switching has to be built into the character. Unless it's possible to create an injector for that.

Sure you can parent detached parts. You can also do it with conformers, basically still parenting. You can blend with transparencies. It's a bit wonky but it's been done a lot. Think the creature creator feet for V4 from daz. Numerous examples over the years.

You can animate joint centers now. You can pass this stuff along in a pmd inject pose. I think that was introduced in 11 pro. You can get 11 pro very cheaply. There's a ton of quality of life improvements since you left. A dependency editor that let's you create complex ERC/valueOps ( also shareable via pmd pz2) within Poser itself. Export morph injection files. Etc. 

You can rig facial parts in Poser itself. Body handles for rigged parts, etc. Look at La Femme. Again, Poser 11.

Weight painting...

Rig your figure as you would the old way with sphere zones and then use the merge zones to weight maps command. You can then further edit the maps using the weight painting tools in the joint editor for each rotation to your satisfaction. You can also paint smooth scaling. Scaling between body parts is much improved now. It's all in the manual. There's also smooth translations now.

There's also an adjunct to the set up room called the fitting room where you can run simulations to more automate fitting conformers between figures

Subdivision and subdivision morphs is also a thing now. Work on unimesh figure stuff. Improvements to the morph putty tool (morph brush)

How's that for a start?