Putting wall bracing up before sheetrock. What is the width of the wall brace for the microhood and how far below the upper cabinet should it be installed?

You can try checking out the installation manual for an idea of how it was installed. It should be sitting on a mounting bracket and not attached to the sheetrock itself. However, you might be pulling on the mounting bracket when you tip it forward. You should have an extra set of hands to help you pull it straight forward and out, instead of tipping it.

The installation manual can be found at https://d1vofmza27mmhi.cloudfront.net/product-pdf/amana-amv1150vaw-installation-instructions.pdf.

Good luck!

parts available here
KitchenAid Microwave Model KCMS185JSS 3 Parts

It is much easier for two people to remove the microhood. If attempting alone, please move the range out so you can get under the micro to support the weight. If the range stays in place, protect the top from damage.
1. make a landing site to the side, to protect the floor when putting the micro down after removal.
2. Disconnect the power cord in the cabinet above.
3. loosen the two screws in the bottom front corners of the cabinet above. When removed the front of the micro will drop, be ready for the weight. Busy times, tools in hands microwave coming down in front.
4. tilt the front down. pull the cord out of the upper cabinet.
5. with micro still tilted down in front, lift the back of the unit upward to free it from the wall plate.
6. remove the fasteners in the wall plate and remove.
Congrats, begin celebration.

Don't know right off the top of my head, but the installation instructions should tell the min and max height from the stove. If they don't you may want to contact an electrician to find out.

It is much easier for two people to remove the microhood. If attempting alone, please move the range out so you can get under the micro to support the weight. If the range stays in place, protect the top from damage.
1. make a landing site to the side to protect the floor when putting the micro down after removal.
2. Disconnect the power cord in the cabinet above.
3. loosen the two screws in the bottom front corners of the cabinet above. When removed the front of the micro will drop, be ready for the weight. Busy times, tools in hands microwave coming down in front.
4. tilt the front down. pull the cord out of the upper cabinet.
5. with micro still tilted down in front, lift the back of the unit upward to free it from the wall plate.
6. remove the fasteners in the wall plate and remove.
Congrats, begin celebration.

the link below will take you to the installation instructions if needed.
It is much easier for two people to remove the microhood. If attempting alone, please move the range out so you can get under the micro to support the weight. If the range stays in place, protect the top from damage.
1. make a landing site to the side to protect the floor when putting the micro down after removal.
2. Disconnect the power cord in the cabinet above.
3. loosen the two screws in the bottom front corners of the cabinet above. When removed the front of the micro will drop, be ready for the weight. Busy times, tools in hands microwave coming down in front.
4. tilt the front down. pull the cord out of the upper cabinet.
5. with micro still tilted down in front, lift the back of the unit upward to free it from the wall plate.
6. remove the fasteners in the wall plate and remove.
Congrats, begin celebration.
http://www.managemyhome.com/mmh/lis_pdf/OWNM/L0211226.pdf

Most Kenmore microhoods use a 3" by 10" exhaust duct. Check for correct demensions. Make sure that the existing vent is this size or that you have room to adapt to this size. There should be a damper mounted to the top of the micro which will attach to the vent duct work. You will need an outlet in the cabinet above the micohood. This should be on a circuit of it's own. The microhood should have a wall plate for the back wall. I'm going to assume there are no templates available. Hold the wall plate against the back of the micro to determine the height placement in the opening. Most install centered under and against the cabinet above. Not all do, so this will confirm. Mount the wall plate to the existing wall studs with at least 1/4" by 2-1/2" lag bolts. Bore an 1-1/4" hole in the right rear corner of the cabinet above, about 2" out from the back of the cabinet and 2" in from the side, this is not a critical measurement, it is to pass the electric cord into the upper cabinet. There are two screws that support the front of the hood. I believe they are 1/4" by 3-1/2" long. If not included, check for size. The hardwae will have them, get flat washers to fit both also. Measure from the back of the micro to the mounting holes in the top of the micro, include the wall plate in this measurement, they'll be about 10" out from the back and about 1-1/2" in from the edge, near the front, but check that measurement, they vary. Drill 3/8" holes in the bottom of the cabinet at these locations. If doing this alone, best that you move the range out. It's nice to have a helper at lifting time. With the plate on the wall and the holes in the bottom of the cabinet, your ready. Put the 1/4" screws and someting to drive them in the upper cabinet, so they'll be available. Lift the micro into the opening with the front tilted downward, hook the back of it into the bottom of the wall plate, push the cord into the upper cabinet, tilt the front of the micro up until you can start the front support screws. Start both, don't let go of the micro until you are sure that both screws are started and will support the front weight. Once you're sure, tighten both screws all the way and connect the vent. Steel tape is best for sealing up any leaks. Power up the outlet, plug in the microhood and put in the popcorn. Get a good movie and enjoy in your accomplishment. Congrats. PS: Make sure the roof vent opens when the hood is turned on, I have seen them stuck shut from grease build-up.

Why can't you use the wall plate? Even if you had to use construction adhesive it would be better than nothing. If you are installing in an island setting then you will have to reinforce the cabinets as they would have to support the weight of the micro.
Eric

Put padding on the floor near by for a landing zone. Disconnect the power in the cabinet above. There are two screws in the upper cabinet that support the front. There is a wall plate that supports the back. When the two front screws are removed you will need to be supporting the microhood or it will fall. A strong assistant is nice to have. I like to have the stove out of the way so I can get under the microwave to support it. Once the front two screws are removed, tip the front down, pull the cord out of the upper cabinet. Then lift the back slightly to remove it from the channel on the wall bracket. Set it down Done.

This is a good question to ask at the point of sale. The measurement varies for different models and makes, but 33 - 36 inches is ideal. If you have a 12 or 15 inch tall cabinet above the stove, you are fine. A microhood does require a dedicated electrical circuit. When powewring off the current hood circuit, check to see if any other lights or outlets go off.