I haven't read and digested everything on this page but this question evoked something in me that I want to get out. I know you're aware that not everyone is onboard with the ideology of non-violence and a lot of my answer is going to focus on that, so this might not be very helpful to you, but it is an authentic anarchist response:
Can they? Well sure, it's possible for anarchists to do any of the three things you suggested. Will they? Or should they? No, they won't and I don't think they have any moral obligation to, and I also don't think you need to be concerned about anarchist violence 'delegitimising' your protest.
Maybe that's because I think moral obligation is limiting and contrary to the goals of my anarchism, or maybe it's because of my general disdain for the cult of 'non-violence' and how it sublimates the rebellious energies of anger and frustration into something that doesn't challenge or transgress authority in any meaningful way and instead becomes part of the supporting apparatus of the state. It's probably both if I'm honest with myself.
Non-violent protest is 'legitimate' because the state legitimises it. It isn't disruptive to the status-quo, it's a safety valve that the state is happy sanction. So what if your protests are 'delegitimised' by anarchist (or non-anarchist) violence? The powers that be weren't going to acquiesce to your demands (because why should they?), and it's not going to effect some kind of sea-change in public opinion on the matter unless there's massive police brutality at your 'legitimate' protest to evoke sympathy (which there won't be, the state learned from the civil rights struggles in the 60's), because no one sees a placard on the news and thinks to themselves 'oh damn, thousands of people disagree with me, I need to change my opinion'. Now that we have mass communication you can always find forum in which your existing ideas and attitudes are affirmed. Non-violent protest (without an external implicit threat of violence) is only good for one thing - making you feel like you've done your bit and that you're part of something bigger than yourself.
I'm pretty young, but I've witnessed a lot of non-violent protests lead to nothing. When the country I live in decided to join the US in its invasion of Iraq millions of people took to the streets with placards and loudspeakers and chanting and all that stuff, but the government participated in the invasion of Iraq nonetheless and was returned to power at the next election. I've also been to protests where anarchists (and police agent provocateurs) committed violence against property and nothing changed; I'm not saying that anarchist violence at protests is particularly effective either. The only protest that I've seen amount to any real change were the London riots of 2012, which started as non-violent protests over a police shooting of a black man, quickly escalated to violent but explicitly political protests and then turned into a city-wide riot. When it was all over the government quietly pressured the police into curtailing their racial profiling and stop-and-search policies.
I just want to emphasise that contrary to your assertion that black protests for social justice are turned violent by non-black people, the London riots were planned as peaceful protests by groups that were predominantly black, and escalated into rioting by predominantly (but not exclusively) black youth. The point here being that in the one case I've seen protesting change anything at all it wasn't because the protests were peaceful, and it wasn't because some anarchists crashed the party and smashed shit up, it was because the state was pushed into a corner and understood that unless they addressed at least part of the source of the rage that lead to widespread property destruction it would happen again. The moral of that story is that political protest does not alter structures of power but ontological rebellion (a rebellion of being and action rather than thought and speech) can. Of course rebellions of thought and imagination are a necessary precursor to ontological rebellion, and ontological rebellion doesn't have to be violent. Just don't confuse marching and chanting (expression) with actually changing shit (doing).
So to conclude, I don't want to tell you how to feel, if you're angry that outsiders are crashing your protest, that's fine, I won't tell you that you shouldn't be angry. However I do think there's no need to worry that they're making your protest less effective, because it was never going to be effective, that's why it was allowed to take place. I also think it's worth thinking about how participating in the the spectacle of sanctioned dissent may reinforce what you're trying to fight, and how you might try to fight what your fighting by transgressing, transcending or subverting it, rather than just expressing your opposition to it.
Sorry if that came across as a bit rant-y but it's an emotive subject for me.
Edit: Also sorry if I came across as condescending, I don't intend it that way, I'm a work in progress.