The Yolanda dress is lovely but a bit short and as you say, might be too hot. The second dress is lovely and seems perfect to go from ceremony to reception to dinner.
Don't stress too much. I've been to lots of graduation ceremonies and no one really takes much notice of the relatives unless they are wearing something startling. Aim for relaxed formal, not wedding level.
Personally, I wouldn't wear ankle boots with a dress - I'd wear flat or low heeled shoes, but that's your preference.