The cup usually creates a higher humidity level (similar to a greenhouse). You can do it, but regularly lift the lid and let in some fresh air. Aside from that, it looks like outdoor.. might be a bit late for that 😉

Glass will not blow away and protect the seedling from the bad weather. This will only work for the very short term. You should find a better way to protect it and let it have some air flow.

seedlings don't need 100% RH so this isn't helpful. The vpd will be too low even for a young plant and only slow growth.

I think it's totally fine to use glass instead of plastic. For outdoor, though, as Papa_T mentioned, it's a bit late. And for me at least, it's also so humid that it doesn't need a dome. But I personally always think experiments are great; it doesn't cost anything outside. If you want to expand it, you can always put a small greenhouse over it and add a light; heated, it can be fun even in winter ;)