I'll be honest with you as I used to raise this breed in many colors. Along with several other breeds. The eggs from the Muscovys had the highest hatch rate of all the breeds I have worked with. I have hatched hundreds and all of them hatched at 28 days. I will admit I have had a few here and there hatch a day early or a day late and maybe 1 or 2 up to 2 days either way. Just to be safe I would give them til day 35 before I toss them. Although maybe some wont agree, I will give you 2 other ways to tell if they are alive with doing little to no damage to the chick.
1.) Get a chicken egg (test egg) out of the fridge and put it in with them for several hours. Then remove it and 2 of the ones in question at the same time and set them on a dry towel at room temp. Wait about 5 to 10 mins and pick up the test egg and notice how cool it feels now pick up the other. If they are dead or undeveloped you will notice they feel about the same temp as the test egg. If alive they will feel about the same temp on the air pocket end but below that they should be noticeably warmer. If warmer immediatly place back in bator and wait for them to hatch.
2.)This method only works with water fowl eggs do not attempt with any other type of fowl as it will kill them. Take a thermomater and a large clean bowl and fill with water. Make sure the water temp is the same or as close as possible to the temp of the bator. (Tip: the bigger the container the slower the temp drops and the smaller the surface area the slower too) Place a few of the eggs in the water. Floating eggs can mean they are dead and decaying and full of gas. Slow sinking eggs are usally good. Do this no more than 30 secs and watch for the eggs to begin wableing and moving. These contain live chicks. They are not sruggling because they are drowning but are swimming birds and have naturally noticed the change in gravity and are trying to swim. I have done this many times on ify eggs and it has never affected the hatch. Keep in mind that in the wild most waterfowl nest near water for protection and go into the water to feed bringing back water to the eggs and somtimes playfully splashing them by accident. If not handled a great deal they should also still have the waterproof coating given by thier mothers feathers. Of course good eggs should go straight back in the bator. Dab dry with a towel. Do not rub dry.
Only attempt these methods when ready to give up. I hope this helps.