Chickens will drown if the humidity has been too high for the entire incubation period. They need the high humidity during hatch. All eggs are supposed to lose a certain amount of fluid during incubation, low temps or high humidity prior to lockdown prevent the optimum fluid loss, but if you've kept it at 99.5F and under 70% up until now, it should be okay.
Not right now, leave the temp at 99.5 and don't open it to prevent temp and humidity swings. You may have to vent some after it hatches because the humidity will increase, but unless it's fogging up the whole thing or over 85%, you should still be fine. (If the other one pips because it's a chicken, you can lower the temp to 99.0 if the humidity is really high to help mitigate it.) I lower my cabinet hatcher to 98.5-99.0 when I put them in lockdown, but that's from experience with my breeds and equipment, I can't give you an exact answer for yours.
(Never raise temp above 100 unless it's a breed-specific requirement. Temps above 101 for extended periods can kill an embryo or cause defects.)
Once it hatches, and that still could take 24-48 hours before it does, leave it in there so the other one can hatch. It can survive for around 72 hours on the absorbed yolk. A chick peeping and nudging the other eggs signals the other ones to hatch.
I looked up Muscovy and they can take up to 37 days, so there's a pretty good chance you've got at least one chicken there. If for some reason the other one is actually a Muscovy, you should be able to let this one hatch and remove it quickly after waiting to make sure the other one hasn't pipped, and keep on trucking until day 30 for the real lockdown. Again, I'm not versed on Muscovy, but other eggs can withstand a day or more of not rotating after they're far enough along in development, so you can put the rotator back in and go from there. The chick knocking the other egg around in an incubator that small will keep it from sticking anyway.
I've had chickens sneak eggs in duck nests more than once, it happens; so if you trust your source, give them a little grace and ask for a refund or replacement. If they're both chicken, I'd question your source a little more though.
Make sure the temp and humidity is stable and then go get your brooder set up for a chicken, make a soothing drink, and do something to keep you from hovering over that incubator. The longer you worry over it, the more likely you are to open it when you shouldn't. We've all done it. It's okay to check on it, but put your hands in your pockets.