I agree bots are an issue, for these and other issues, the most important of which, in my opinion, is because in the entry leagues they make the new players experience very frustrating. While the leader board issue without CP should be indeed addressed, I do hope developers will also take action regarding lower leagues. For making player eperience less frustrating, one option I see is that every account should self/declare human or bot (which would not be allowed to change during a season), and every player may chose either to battle against any opponent or to battle only humans (thus also accepting that matching waiting time could be longer). Of course, other options are possible too.
By the way, you state that those accounts in the LB "...are either bots or humans who played with rental cards, but do not have the cards at the moment" - I suppose they could also be humans with own CP that decide to make use of the high rental prices during EOS and put their cards to rent after they guarantee LB reward, couldn't it be?
your second point can easily be verified. Blockchain is public...you can easily check which card anyone owns... go ahead and check this link below :)
https://peakmonsters.com/@zeakio6/cards
These accounts are a DRAIN on the system. I call them LEECH.