EEschema and the underlying concept is a bit worn by now (probably 4 years old, minimum) and the devs currently work on getting it up to the same philosophy and state of art as PCBnew. When the current rag-tag-system was put in place over the years no one wrote a roadmap.
No need to use those words.
Can't help ya with the "Select Footprints to Browse" function.
My custom fps have no keywords
But I also can't deselect a library, so you're not alone..
The normal way is to use CVpcb to assign footprints to schematic symbols.
As you're using the tool in a way that wasn't intended (might even be the first to do it that way afaik) you run into problems, no one else has encountered.
This way I have no problem to get any footprints listed that have 0805 in their name from any library I got.
Then just doubleclick the fp on the right and it get's assigned in the center to the currently selected REF_ID.