I use it quite an exceptionally large amount at work (school tagging lists are… extensive…).
No clue what a checkbox group is, I don’t recognize that W3 terminology.
A multi-select form is a multiple-selection form. I tend to have them be size=10 or so high, depending on the usage context. They’ve never looked bugged out to me? The browser renders them as it sees fit.
Depending on the browser, I’m unsure if the internal elements can be reliably styled like that as sometimes they just default down to the standard operating system render of it.
Those aren’t called anything like “checkbox groups” (at least in W3 definitions), those are just loose checkboxes within a form. Those are pretty trivial to do. Why not just render a list of those?
Those are just loose checkboxes, not a multiple selector or so.
A single value or multiple values? I’d imagine they would have to be multiple values since there are multiple items, though you can easily pack those into, for example, a map or so via adding [] to the ID’s.
Radio Buttons are a group of checkboxes that are single select, not multiple, that’s the radio part of it, from old interfaces. 
That’s a fieldset then, and it doesn’t do any inherent grouping of value keys.
Yep, this is what I was referencing with the [] above, Phoenix will autocollect those into a map, it’s quite convient.
Especially with a lot, multiple select is so much easier on mobiles especially.
