I once saw a mason take a dull non-carbide blade mounted backwards in circular saw and cut out mortar. I wasn't there for entire job, he may have had to use more than one blade. Use an old saw if you have one, lots of abrasive dust, protect eyes and lungs. Take breaks to prevent blade from getting too hot. Carbide teeth are liable to break if run backwards.
If you do not have power impact chisel, masonry bit with diameter slightly smaller than mortar line will go thru mortar quick. Series of holes, knock out mortar between with hammer and cold chisel. Mortar behinf brick may be holding it in place, try a few relativly light taps on it, see if it breaks free. You might have to break a brick with hammer and chisel to get it out, then you can probably get chisel behind next brick. Once you get a couple out, others shouldl be easier.
Only "load" I can figure is lower part of chimney protruding into room is bearing upper chimney. So take a look in attic and above roof compare dimensions.
Measure twice, cut once.
Look at the nail, not the hammer. Watch the fence, not the blade.
If you hook your thumb over your belt you won't hit it with the hammer or leave it layin on the saw table.