The last few years the summer have been a lot hotter than they have in the past, but it's still bearable. While you're at the Fort, the heat won't be that bad because of the shade, it's more while you're at the parks that you'll have to worry. Just be sure to bring lots of water, and those little portable mini-fans will save you numerous times while in the parks.
Normally it will rain in the afternoon for an hour or so, so it will start to cool off while that builds up. Of course, then you have to deal with the humidity that comes after the rain showers, but all in all, it's not THAT bad. Lots of people make the trip and make return trips and survive...lol
And yea, bugs aren't that big of a problem, just bring some spray and you should be fine.