Regarding the Antifa and BLM riots, do you think defense "it's happening under Trump" that many on the left use is valid?

Sorry if I'm misinterpreting, but your comment is a bit unclear. Are you asking what I think of people excusing rioting with the defence "it's happening under Trump"?

I haven't seen much people make that argument, but yeah, it's a bad argument. Rioting isn't the solution. Mob violence is always bad. I think that "many on the left" would agree. Maybe we hang out in different areas of the internet, but I've seen unanimous disapproval of rioting, even among people who support the protests in general.

If so, say that Biden wins, and Trump supporters start rioting every single day - burning down buildings, looting, killing people, taking over U.S. territory and claiming to have seceded from the union, etc.

If Trump ran again in 2024 (or Don Jr.), would you think "these Trump supporters rioting is happening under Biden" would be a good reason to vote for Trump?

I'd say the exact same thing - I fully support their right to protest, I fully condemn rioting. There are plenty of pro-Trump protestors right now and I fully support their right to protest (even if I disagree) but if they get violent, that's where my support ends. I would be willing and able to distinguish between rioters and peaceful protestors.