I'm not familiar with your editor or block-based editors in general, but it looks like a basic version of a visual editor I used in WordPress a while back. Is there some reason you can't do it similarly to how that did it, by allowing a row to be split into columns, each of which can have a content element within it and can have it's width adjusted manually or set to fit to its content?
If you used flex instead of floats it seems like you could have most of the functionality without the edge cases of floated elements.
CK is our workhorse for tons of sites, and we've struggled to come up with a good general pattern for editing horizontal layouts. We'd happily take suggestions, however. I'll admit it's one of the more awkward things with the editor.