1 - This is not tanking assuming you are making quality trades to obtain future assets (draft picks) or high upside guys that aren't currently performing but are likely to in the future. This is building your team for the future and the byproduct is that your current year team is diminished (which also helps for your future).
2 - This should never be done in any league regardless of the rules. This is just bad ownership and I would never want to be in a league with someone that would do this. It affects the integrity of the league and should never be done regardless of the rules.
3 - Same as #2. This is terrible and is irresponsible ownership. I would not be in a league with owners that do this.
Owners should always play their best starting lineup to try and win. If this doesn't happen and you purposely play inactive players then you should be removed from the league. It affects all other owners negatively that are fighting for playoffs or titles and at some point this method will come back to bite you if you are fighting for playoffs or a title. The integrity of the league falls apart if teams are purposefully trying to lose whether there is a specific rule banning it or not.
1. Ask a Marlins fan if what Jeter is doing is tanking or not. I get what you're saying, if you're trading good pieces to obtain future assets and trying to better yourself then it's being productive, but it's still considered a form of tanking IMO. Trying to lose in the short term but trying to win in the long term/future term.
2/3. Every single league in existence should have a rule to the effect of "you must start a complete lineup, no bye players, no IR players, no blank spots". Stuff happens, lineup changes made don't get saved right, Sunday morning emergencies prevent someone from making a roster move, but every league should have such a rule in place. That takes care of half of this problem. The other half is, if I'm insistent on tanking but abiding the rules, then I trade Gronk but pick up and start the Browns 3rd string TE who only plays special teams and gets 0 points every week. I'm within the rules (starting a healthy active player) but obviously I'm not trying to get any points because I chose this player instead of actually trying to find a waiver guy like last year's Kittle/Njoku/ASJ type guy. Unless you have a set of rules specifically crafted towards how to handle this half, then you need to have a conversation and an agreement amongst the owners of how to handle this.
I wrote this in the other SP thread, but I play in a 12 team keeper league where we allow "tanking", but we have rules in place. One of our rules is that if you're going to sell your good players for draft picks, then it has to be open auction to all owners. No going to your buddy and offering Gronk for a 7th. We have to start a full, competitive lineup. And our consolation bracket standings determine draft order, so toilet bowl winner gets 1st pick, 2nd gets 2nd, etc. So even if you could just "give up and bomb" style tank, then you'd pick 6th at best.
As to Gally's bolded text above - I 100% get what you're saying. Maybe it's a personal opinion thing with me. But in a keeper or dynasty, if we're at Week 6, there's an owner who's 0-5 and decimated with injuries (no good outlook ROS), I'd rather he be able to "tank" - strategically play out this season within rules but actively try and better himself for the future - rather than just set a healthy-but-lame-duck lineup and not pay attention because he's out of it. In a true redraft, where nothing about this year affects next year and every year the draft order is randomized, then yes, I agree, everyone should try hard to win every single week, but then you need to monitor owners who give up and consider replacing them for owners with more integrity.