I can't figure out the purpose of collections and series.
To the best of my knowledge, the Contributor has no control over which images show up in the series in the Buyer portal. A collection can be used for self-promotion, as well as a way of keeping track of which images you've already submitted,
It's a bit odd that collections aren't used even though they can be potentially much more accurate than a series.
Series are quite accurate, at least at my portfolio. I suppose there is some keywords, some visual mix for this.
1) no. Adobe uses some algorithmic magic to do the matching. Assets don't need to be uploaded sequentially to make part of a series.
2) Collections are for you organising your assets. other than that, they have little value.
It's unfortunate that the contributors don't have any control over this. The hard work to create a unique visual and offer some variety for the buyer go a little bit to waste when the matching doesn't work. The upload order seems something that has some correlation, especially when a series had very visually distinct assets.