A few things I noticed when you want to make use of the new tools and are trying to export the complete character at once (please correct me if I am wrong):
Regarding the points you raise, here is a little feedback from the CC3 dev team.
1: It is not possible to reduce the triangle count of the body mesh without combining it to the wearables.
Currently the only way reduce the body mesh is convert to our game base now using the option to "Convert to Game Base" on the Modify Panel.
2: It is not possible to combine meshes without reducing triangle count.
Right now we only have a way to combine mesh, you should convert the assets to Accessory and select the Accessories you want to merge, then go to Menu - Modify - Merge Accessory and execute it.
3: It is not possible to export the clothed character without the body.
We will consider to have this ability in a future update.
4: Every InstaLOD has its own baked texture set.
Yes, but you can choose to bake or not bake.