From what little I know & can say from my perspective only:
The connection caches are a little different. Also, LW 5 advanced their connection technology by self-seeking & finding fresh ones to update the cache with, whereas BearShare does not have such an optimized system. LW 4 suffers a similar issue as BearShare.
Likewise my BS connects to 16 UP's in general, so I don't think it's anything to concern about. I find my Bearshare 5.1 beta connects to other BearShare UP's mostly, which IMHO is preferred. The LW 4 UP's tend to be spam UP's or connected to a lot of spam users corrupting searches with rubbish results.

There are other BS Pro or 5.2 users out there who appear to connect well as my connection window shows.

I have no idea why when forced UP is turned off, BS struggles with connection, but quickly succeeds when forced UP is re-enabled. I'm sure someone can suggest the reasons about this & your other questions.