Law mower screw stripped and blade can't be removed

Usually bolts on mower blades are left hand bolts so you need to turn to the right to loosen.

check the belt tension that blades needs to work the most

Sounds like a problem I had, and it was the primary gear on the motor was stripped. So far no luck at finding the gear, but to replace the trimmer without battery would probably be cheaper.

As for the stripe, you will need to change the blades, they are worn out, if not damaged outright. As for the cutting the grass lower on one side, since you are changing the blades, remove the deck and inspect the spindle/mandrel on the troublesome side paying close attention to the ears that the bolts that mount it go into, you may well find that one of them is cracked. If so, replace it. The spindle/mandrel cost a little over $60.

I hope this helps you out.

I use a piece of 2x4 . Tip mower onto it's left side, stand 2x4 between deck and blade and remove bolt. You will need to use your leg to keep mower from rolling back. This is just a simple and inexpensive way. You may also buy a special tool to hold the blade while you remove the bolt.
GOOD LUCK>

There are two philips screws on top. I got them with a screw extractor as they were rusted. You won't be able to fasten the blade from the motor side though.

First, you need to make sure you have the right tools for the job. You'll need a wrench, work gloves, mower blade clamp, drain pan and fresh fuel. Check your owner's manual for the correct belt to your specific lawn mower. Belts that are not specifically designed for your mower will not fit properly and may wear down prematurely. Next, remove the spark plug wire. This is a safety precaution to prevent the mower from accidentally starting.
Now you can remove the screws that secure the drive cover to the mower deck. Once you've taken off the drive cover, loosen the screw that holds the belt tension spring to the transmission. As the screw is loosened, tension on the belt is released allowing you to remove the belt off of the front transmission pulley.
At this point you'll need to turn your mower on its side to take off the mower blade. Be sure to use a work glove to protect your hand when removing the mower blade. You can use a lawn mower blade clamp over the blade to secure it. Unscrew the bolt to remove the blade and then the engine baffle. Once the pulley unit is exposed, you can remove the entire drive belt. Insert the new drive belt and place one end on the rear pulley and re-attach the engine baffle. Then, place the mower blade back (making sure it's facing the correct way with bottom of the blade facing the ground) and tighten the bolt. Remove the clamp and return the mower to its upright position so you can place the other end of the belt to the front pulley. Remember to tighten the screw to the tension spring.
Put the front drive cover back on, tighten the screw that secures the cover to the deck and reconnect the spark plug wire. Add new fuel to your mower and you're done!

Here's the scoop on the MM875: it has a plastic piece called a "blade insulator" (part # 241381-01) that "locks" the shaft to the blade, so that you can remove the nut by stopping or blocking the blade. If it has been damaged or stripped out, then stopping the blade won't do anything. However, the metal spacer directly under the nut IS still locked to the shaft (because of its shape), so you can use a pipe wrench or something to hold that in place and unscrew the nut. You might even be able to get someone in a home improvement store to do this for you. But you will still need to replace the insulator, which is only a few dollars.

Bet on it- you might try adding an additional nut of the right size/thread or several washers to utilize the remaining healthy threads.

you find the right size socket that fits it.(it: the bolt in the middle of the blade under the mower. Then hold the blade as you turn the socket wrench, sometimes this will be hard. this is how to do it