This one is not a super strong anti-pattern, but sometimes it's there. A good example of this would be a 500 damage nuke that slows enemy attack speed by 50% for 10 seconds (as opposed to say, 20%), on a 20 second cooldown. At 50%, this is a strong combat initiation disable... but at 500 damage it's a great finisher on someone who is running... but you also want to use it early to get the disable -- even though you won't have it avail by the end of combat usually to finish. This makes players queasy about using the ability much like in the optimization case, but it's a slightly different problem. If the ability exists for too many different purposes on an explicit basis, it becomes confusing. this is different from something like blink which can be used for many purposes, but has a clear basic purpose -- in that place, players tend to just feel creative instead.
I agree with most of your list, but I have an issue with this one. The choice that you mentions sounds pretty interesting, and I think the tradeoff between power and utility in spells is something that makes characters much more diverse, complex, and satisfying. Not to mention this is all over the place in your game, and it's totally fine as is. : P
A great example is Annie's passive in the laning phase. Her Q is great for getting CS, but once you're at 5 stacks and ready to stun, you face an interesting choice - do you continue to CS with Q, ignore your passive for the time being, and just keep farming? Or do you make use of the stun to harass your opponent with a potential Q --> W combo? We're talking about a big difference here, farming gold at 0 mana cost vs. harassing your lane opponent pretty heftily, but burning a good bit of mana and over-pushing your lane because of W.
And yet, it's a fascinating trade off and makes Annie legitimately interesting.
So, don't worry about "conflicted purpose" too much. It's pretty fun imo, just about every champ in the game has it in some context, and what players decide to do with their conflicted purpose spells is what separates good players from great ones.