Without using the rain fly, do the windows zipper up all the way so that someone (or something) outside of the tent won't be able to see in?
The Solitaire's rain fly is attached, so you can roll back the yellow (and some black) sections. What's below is mesh - and at the top, this means there's a zipper to escape out from the middle when the fly isn't engaged. There's not much privacy when all of the individual pieces of the fly are rolled up.
The tent zips up all the way, but a person outside the tent would be able to see in.
No, the ultralight design only covers some windows with the fly. But without the fly the view is great!
When you remove the rain fly (which rolls up & stays attached to the tent at all times), and close the zippers all the way you can still see inside the tent. Without the rain fly, it offers very little privacy.
They do zipper up all the way, but you will be able to see in (and thru the tent) without the rain flys. The flys are also permanently attached, so there's no way to make this "uber light" if you're looking for just bug protection. Worth the money/weight to me when they are on sale, I got three from Sunny's.
Something to consider if you're looking at backpacking with this a lot for about the same weight is the Black Diamand First Light when on sale. In addition to being about the same weight, you can fit two people and no gear, or one person with gear, you can sit up in it, and it's four season. It's the tent I carry now. I got two from Sunnys. I still use my Solitaire, but just as a "bug screen" when sleeping on a cot at Scout camp etc.
I have used the Solitaire in a NOV rain/sleet/wind storm at Dolly Sods and it kept me dry, just not able to sit up.
By "windows", perhaps you mean the top part with the zipper along the top? If yes, then the answer to your question is No. The Solitaire is almost entirely mesh and without the rain fly everything in the tent is visible.
Ventilation is not great, despite being almost completely mesh, because there is no "high" venting since the rainfly covers the tent almost all the way to the ground.
Without using the rain fly the zipper can be closed all the way and screen like material will allow air in and can be seen from the outside.