I don't see that adding windows would be a problem. To wit: the sink it very oddly placed, and in characteristic with the rest of the interior, apparently impossible to actually use effectively. Stealth camping requires you to be able to function inside the van, but this is like the opposite of every design aesthetic and ergonomic consideration I would personally apply.
Consider a moment standing up from one of the benches- with the extended outreach of the shelves overhead, for an average height person it might be very difficult to simply stand up normally as there is an extended shelf very close overhead. This shelf overhead would seem to require leaning forward to clear the shelf during standing, which seems in itself acceptable, however the identical shelf opposite prevents exactly that. What is left is a very small vertical space to complete the action of standing normally.
It's almost like someone designed around a sitting person and a standing person but didn't consider moving between those states. On first blush, I'd say the low miles are from the torturous confines of the design, but I'm nothing if not persnickety about my space and would be very very uncomfortable in this van.
So, my personal opinion would be a total tear out and build from scratch- in which case, adding windows would be no problem.