First question: Yes. Attribution is still required, even on stream or youtube. However, if the youtube is showing "lets play" video, for example, of a game that features the asset, but the youtuber/streamer is not the person who used the asset themselves (the developer of the game is) then it could be considered fair use (attribution only might be required) If the streamer/youtuber used the asset as an add to their video, then it is definitely not fair use and attribution is definitely required still. How they attribute you is more nebulous, but it must be enough for a viewer to know where the asset came from. They could just give attribution in the description, but some authors find that insufficient and insist on on-screen attribution. Not usually on-screen for the entire duration of the asset's use, but I guess you could insist on that. Generally just on-screen long enough for the credit to be read by a reasonable person. It is your right as an author to set those thresholds, but 1) you should be clear what you expect up front, and 2) understand that the more restrictive you are with attribution on atypical mediums (like videos), the less people will want to use your content.

Second question: I had previously given (in discord) a breakdown of the steps I would personally go through to ensure my conditions were being adhered to. Let me see if I can find it again. Basically, always start polite. Thank them for their interest in your work and ask if they are willing to make an adjustment. If no response or inadequate response, insist and link to where such requirements were already outlined for them when they decided to use your assets.

If you'd like, I can be the annoying person for you. It is something you should insist on getting credit for. Attribution is the price of using the asset. When someone uses a product without paying the price, it's called theft.

I don't want to be that annoying person badgering everyone when they fail to provide attribution or do so incorrectly,

I used to feel this way, not wanting to make my attribution requirements so annoying that people wouldn't want to use my assets. Here's the thing, though; the kind of people who feel it is too inconvenient to give you the credt- but isn't at all inconvenient to use your free assets- are not the kind of people you should want using your assets anyway. You aren't losing revenue if they get huffy and refuse to use your assets. Conversely, you aren't gaining any reputation or exposure when such people use your assets but don't credit you.

@authors: You deserve credit for your work. You may choose to waive the credit requirement, but even that decision doesn't mean you no longer deserve the credit.