stone wall

Hi, I have a slate wall cap 12"x24" on the wall which has come loose. I was thinking of using an adhesive of some type so I don't need to chip off the old mortar or get a grinder with a wheel for doing it. Does anyone have any recommendations? TIA!!

As a disclaimer, I have never attempted a masonry project before and have no background or experience in construction. If this is better left to the pros, feel free to say so! I have a stone porch from the 1920's which has been patched with concrete (?) over time, making it look messy. I hope...

I am bringing a 17?? stone house back to life. the walls are entirly made of stone and are .8 meters thick at some points. How would I proceed with installing a window, without risking of bringing down the wall. regards.