Yea, this is just "one of those things". You can specify it in your rules, however I don't. Because sometimes sh!t happens and you pick up a guy and then 3 hours later news comes out and you would rather have that new guy and you drop your newly acquired player for the newer hotness.
In my league this would probably never happen since we charge $5 per transaction. That's a lot of moola for 6 TE's. Having said that, if your situation happened in my league I would also unlock all those TE's based on our owner Conduct policy and for a fair league. While some leagues may see this as a "shark" move, we see it as bush league (as do you). If for some reason the owner thru a fit about it then I'd toss it over to my competition committee for a ruling.
As for #2...there actually is a setting (which I don't recall being there before, but then I've never looked for it) under Waivers/Free Agents setup that is:
Once a player is acquired, he can't be dropped for this many days...then select the number of days with 1 being the minimum.
This wouldn't completely eliminate the issue if you had unlimited roster spots but I think the guy wouldn't be able to do this en mass unless he had enough roster space. The downside is that it prevents an owner from changing his mind or perhaps making a last minute move before kickoff if say he picked up a guy on Saturday and now on Sunday he's declared Out and the owner would rather drop and pick up someone else. So you'll have to decide if you just want to police it in the rare instance it happens or deal with less flexible rosters overall.
I think your post may make me update my rules to just note that this type of thing is not acceptable. Amazing that even after 20 years I'm still updating my bylaws.