Lists can be renumbered if they get out of sync, by selecting the list, right-clicking on it, and using the “Re-number List” command.
Something you can do to help avoid that happening in the first place is using the shortcuts, found in the Edit ▸ Move submenu, to move lines up and down, rather than cut and paste (if that is what you are doing). These work for normal paragraphs as well as list lines, and they should re-number automatically when doing so.
Also, having an ordered list preset would save so much time…
Yeah, unfortunately the list tool we have available to us doesn’t have this feature nor a good way of creating one (otherwise we’d probably have done something with styles).
The best thing you can do is make a series of entries outside of the draft somewhere, perhaps even in the document templates folder, with a number of levels already set up with some sample text. Then you can either use the Project ▸ New From Template submenu to add one to your Draft (probably merging it within the previous text item after doing so).
Another approach would be dropping the folder with your sample lists into the Project Bookmarks list in the inspector, so you can more easily access them from everywhere. You can click on the little down arrow button beside a folder in the inspector’s bookmarks tab to select a list type, then copy and paste from there.
I know Scrivener isn’t an outlining program.
Well, that’s a bit like saying OmniOutliner isn’t an outlining program because it doesn’t have much for bullets in its notes field. Scrivener’s “notes field” is of course far more robust than that, but it’s still a better way to classify the software. The outline is one level up from what you’re thinking of.
Of course, I understand that isn’t what you are doing most likely, and that you need these in the text for presentational purpose—but to say it isn’t an outliner at all would be to ignore the genre of software it belongs to. 
That said, I did recently put together a little sample project of how one can create numbered lists for presentation, using Scrivener’s more agile outlining capabilities as intended. Of course using the outliner to outline around text chunks definitely does mean deep diving more into the binder as an outline and not as a series of “documents”, and using the Scrivenings view mode far more heavily. Working more toward that approach does make for some interesting ideas though, like having your figures and tables in separate chunks, making them easier to find on their own, etc.