As many have already hinted (or stated), you simply cannot go outside during the day. Many members here visit during the month of May for the BCA event (I recently made my 7th trip for BCA, 10th overall) and we can all attest to how warm it can be in the Spring. A summer trip will undoubtedly be 100+ the entire time, likely 110+.
With that said, spend the daytime indoors and the go out at night. Like others have stated, I'd just stay at the host location. Even though Vegas has a lot more to offer, the convenience of having a room you're only 2 minutes away from is unparalleled.
If you do choose to stay at another location, rent a car. Don't bother with cabs and definitely forget about buses or the train. Although the bus pass is nice for doing the tourist thing, it's not reliable enough to get you to your match on time. The monorail has a more reliable schedule but it doesn't have a nearby station for the Riv.
My last bit of advice for you is to leave all of your bathroom supplies at home and buy travel sizes once you're there. There's a Walgreens or CVS every other block. Just go there and get everything you need. While you're there, get a case of water to stick in the fridge and bring water with you every time you plan to go outside.