What is a "sleeping bag sleeve?'

The various alternatives you suggest would work for the most part, but not in my view, as effectively or efficiently.

In the past few years, I've always used a bivvy. One among several advantages it offers, is the ability to dispense with a groundsheet.

My particular sack is merely water-resistant on top and waterproof on bottom, so I can't directly address the question of breathability of various materials. My impression is that it's not generally a big deal.

Bivvys are often sold as primarily a replacement for tents. The disadvantage often mentioned is that they are cramped. They are certainly not more cramped than a sleeping bag, which is exactly what they are -- a bag in which one sleeps. Hence I don't understand this typical objection.

They make perfectly lousy tents, but are somewhat better than nothing. I don't view them as a tent substitute, but rather as an invariably useful sleeping bag accessory.