I have lived in one over 20 years and have done several things about heat loss, etc..
A few comments: New insulation code for new buildings: R32 for ceilings. Thats about 9" of fiberglass. Walls is R13.
The walls are divided into 2 or 3 sections in the 16" stud spacing so 2 or 3 holes is requied in each stud spacing to fill all of the spaces. Also the wires, etc in the wall can block insulation filling the walls.
If the walls have thin paneling on the inside it is simpler just to pry a space for a finger hold and pull the panel off the studs. Small finish nails are all that hold them on. Then put in R13 insulation and nail them back on. I would suggest 5/8 drywall under the panels for sound and fire protection.
You must be aware of problems/issues with tankless water heaters. One is unless the water volume is turned up the heat dosen't come on. Another is only one person or appliance may be turned on at once unless it is a large heater. These new devices probably haven't been around very long so their life, actually, is unknown. If the Co. goes out of business before the warrenty is up "tough luck".
The ceiling T&G boards were put on green/wet so they have shrunk and have big cracks between them. A lot of caulking everywhere is important and bugs and dirt falls thru the cracks and air leaks out as mentioned.
Homes with steel radiant heat can and do rust thru and water can be lost, and hot water also, and is repairable at considerable cost and next month a place a few feet away may spring a leak. and on and on. Carpet layers can nail thru the heat pipes and cause a leak. Carpeting and foam pad blocks most heat from coming thru. Only certain flooring will work and not hardwood and the floor should be accessable for repairn leaks. Copper is better ,but slabs develop big cracks on adobe soil and the pipe can break from stretching.
Also the old slabs arent insulated on the outside against the earth as new code requires on new construction.
High effency, smokeless fireplace inserts are recommended and are cost effective for cold winters. Gas prices are high and bound to go much higher as new power generating plants are using up the natural gas. Necular power could have been the answer for elect generation ,but it appears the coal lobby has put that off the table.