I’d go ahead and stay on iOS 9. IOS 10 really is nothing to write home about. I only upgraded so that I could be conversant in it so that I can help people on this subreddit when they encounter problems.
Laketech
Updated my Ultra 1 fine too.
With the Modular Ultra watch face, unless you have the outer ring complication (depth, altitude, or seconds) the notification dot will be off center.
If you use a thin film screen protector (I use LK brand from Amazon) the 40mm screen protector will nicely fit a 38mm watch. You’ll need to make sure you align it carefully when installing them, but it will fit very very nicely, with good coverage around the contours of the screen.
Mine is like 12/months /mo with fees on Verizon, and it piggybacks off my phone plan data. I activated the Series 3 as my original cellular capable watch when it came out. Back then all carriers were waiving the activation charges and giving three months free, ever since then I’ve just transferred to a new watch every year and never had to pay another activation fee.
If she’s having any trouble with her eyesight, you’ll be better off getting her a 45mm model watch. Take it from me, an old guy, it’s much easier for me to read/interact with.
Same price at Walmart Right now.