I'm gonna discuss the points present as-listed, since I can't fathom any particular order would really be beneficial.
In regards to Pre-Game stuff (Pick intent, bans, and lock-ins), the more that I think about it in regards to cases of trolling (or even unfortunate coincidences), the more I realize that it comes close to the argument we use warning players against playing Blind pick if at all possible. Which is to say, Blind pick has no real system set up for Champ Select, and everything - from the team comp to what lane/role you get - is up to player courtesy, and nothing is regarded as a given right.
To the same end, Draft Pick narrows down those problems into something with a bit more specificity - you may now be able to get roles more reliably, as the matchmaking system tracks the position it queues you into and, consequently, role theft can be punished since it is now more of a right than a courtesy, but the problems pertaining to Champion picks and composition are still present, if not exacerbated by the systems surrounding it.
Being able to play a certain specific Champion is still a courtesy - one more readily offered by the community, for various reasons - but with ten bans distributed amongst all players, it is wholly possible that you still won't get to play your pick intent - the enemy could ban them, your allies could ban them, etc.
And while pick order provides a layer of strategy to pre-game, it also presents another problem reinforcing the "pick as courtesy" problem. As you said - if you and your top laner both want to play Sion, but you're first pick, who gets to play Sion is up to the courtesy - do you reserve it for yourself, or allow the top laner to claim it?
In my opinion, though, it may ultimately be a necessary evil. Fixing one problem could just lead to a few more cropping up, and some problems, no matter how they're addressed, may not be able to be fixed. I mean, we've all seen the argument against intent > bans - a troll could hover the FoTM OP new Champ and let the enemy first pick it, for example.
Summoner Spells are something that, IMO, should be looked at on a case-by-case basis, since, as stated, there's no inherent rule preventing specific Summoner Spell combinations from being used, but there will always be a layer of skepticism surrounding them as well, primarily due to the "Disco" combination.
Likewise, while Flash is a ubiquitous Summoner Spell choice, there will always be room for some Summoner Spells to take its place in builds. I've seen Darius players running Ghost+Teleport, Ghost itself tends to be a staple for Singed players, and Supports and Bottom Laners can flex out their spell choice pretty wildly, even beyond the typical fare of Flash+Heal and Flash+Exhaust/Ignite.
Communication, particularly for off-meta stuff, should definitely be closer to standard. I'm totally for new and unique strategies (Ezreal Support, for example), and I'll even begrudge those that I don't understand and wouldn't actually want to see (Deathsiege Sion), but the biggest thing that needs to become standard for breaking the meta is letting your team know that you're going to do something unusual.
And, to that end, communication is a two-way deal. Personally, I think there's room for forgiveness for just saying what you're doing if your unusual strategy is limited to how you play your given Champion (Like taking Hail of Blades on Darius, or Predator on Kai'Sa, or anything else like that), but if you're really gonna go out there with an off-meta pick, especially one that demands a special strategy built around it, you need to communicate with your team, and either see if they're good to work with it, or play something more meta if they aren't.
'Cause, remember, Singed Support wasn't banned due to their strategy, but due to their adamant insistence that they play it and the team adapts to their needs, and not the other way around. Communication and teamwork is a two-way street. Work with your team, not against them.
And, lastly, in regards to split-pushing, it's something that I would say that, like Summoner Spells, should be looked at on a case-by-case basis. There are some Champions that are fit (and even purpose-built) to split-push, but, while the point can be made that their win condition is the split-pushing, it should also be noted that that win condition may not align with the rest of the team's, or even with the current gamestate.
If the entire team is pushing the middle Inhibitor Turret, for example, it's not even remotely close to sound reasoning to prioritize the top lane inner turret over protecting base.
In the case of teammates opening lanes, I think that the remaining players should at least try to flex into the open lanes and defend as normal, though far be it from me to dictate the standard for niche cases like that.
For the most part, those are my thoughts & opinions on the presented matters. One other thing worth mentioning (since my talking about the pre-game lobby was primarily systems-related), is that I feel that pre-game lobby behavior should definitely remain a subject for manual review, though I'm unsure to what extent I would consider acceptable.
I suppose I can put it like this; I would definitely love to see troll behavior in pre-game lobby (malicious bans, pick-stealing, etc.) consistently punished, though in the same turn, I definitely wouldn't want to see my brother punished for his decision to always ban Yasuo - he doesn't do it to spite teammates, of course, but I wouldn't want his match security to be misconstrued for trolling.
And, of course, curse you Pandaa for getting to this topic before I did.
