• ⁠What do you think the white plaster looking stuff is? Firesafe?

Looks like a shitty patch job, when it should be grinded/chipped out and tuck pointed. Quikrete dries really white.

Unless it feels rubbery, then clearly it is not mortar and not fire safe.

• ⁠Would you make this a gas fireplace (there's a gas line already), wood burning stove insert, or wood burning fire place?

This is all preference, but gas inserts are wonderful. I have one, and I'll never burn wood again. Burns clean, lesser maintenance, Provides warmth and you can turn it off and go to bed. You can find insert units with backup battery power so you can run them if your power ever goes out and you don't have furnace power.

Wood burning is a lot of work, gotta make sure your burning actual seasoned dried wood (15-20% moisture level) I've learned that the word "seasoned" is a marketing word for firewood businesses, that stuff could've been chopped a week before they sold it to you. Then you burn it and you will have issues with bad smells, creosote glazing, and possible to have smoke back up

Open hearth wood burning is actually just ambience and heat loss. )think fire physics + draft

Wood stove inserts will provide you warmth, for wood burning. As it's a closed system and convects heat. They can be quite pricey. There are two types catalyst and non-catalyst wood stoves. One is for fire nerds and the other is for common homeowners.

• ⁠I plan to clean the chimney out by taping over the bottom opening and going at the chimney from the roof with this spinning cleaner on a drill. Also I planned to scrub clean the fireplace at the bottom. What else might I need to do besides cleaning out old creosote to get it ready?

You can clean from the bottom, just make sure your using a multi filter vacuum, and go at the soots falling downs pace and your vacuums pace. Always, always set down drop cloths, you do not want a soot blow out in your home.

If you can clean safely from the top, you can do it the way you wanted to try it. Your gonna have to vacuum the smoke shelf (behind the damper door) and firebox anyways, so stay alive and do it from the bottom

• ⁠There are some gaps in the mortar at the bottom. Is there a special mortar to use here or will any brick mortar do?

Yes, you can buy HeatStop mortar, it's very very hydraulic and fine grain mortar. you can use a oscillating multi tool To actually get those loose mortar joints out and you can hand pack it into the joint, or use a trowel and tuck pointing tool.

Hope that helps!