Wasn't the @ also invented by users? I remember it was fascinating to watch this network self organize and create conventions of its own, that are now used everywhere.

Also hashtags. Though I think some of this stuff came from other social networks.

Seems to me like Jarkko Oikarinen or one of his crew invented hashtags, no? Denoting the context of your communication with something like #warez or #hack significantly predates web2.0.

Rather I think Twitter-style hashtags take inspiration from IRC channels in the format of #topic

Because channel names are not hashtags. The syntax is purely because IRC is a text-based protocol, so you need a special way to distinguish channel names from regular text.

# also has the advantage of being on a key on a T9 keypad while using twitter over SMS was in style.

That use— to define IRC channels— seems distinctly different than Twitter hashtags to tag individual posts. I wouldn’t be surprised if hash tags started as a nod to that, perhaps even jokingly, but I don’t think you could consider them a descendent.

I think reblogs as a technical feature were added first in 2009 by Tumblr and slightly later adopted by Twitter - not that it matters in this context.