Radiant supplied with passive indirect tank or on demand or both?

HydroNiCK

Hello all you wise men of thermal comfort and heroes of heat loss havoc,

I am installing a radiant floor in a room over my garage. The room is a split level lofted bedroom. It's 350sf with 16ft cathedral ceiling, windows on three sides, and no subfloor (worst design ever in regards to heating and cooling). The loft is on the 2nd floor. I'm installing radiant in the first floor level which is 190sf. The heat loss for the room is a little over 6k btu. I have a 80k btu non-condensing gas fired Utica boiler from 1968 (The same year my newly moved into house was built) heating about 40-50 feet of baseboard in the 2nd floor apt. above me which my mother-in-law occupies.. The rest of the house is unfortunately heated and cooled with a combi furnace. The furnace wasn't updated along with the new rooms and therefore is not sized correctly. I took the room off the furnace system and added 247 ft. of 1/2 pex 9" oc. with plates underneath the pine floor over the garage. There is no sub floor. I'm having the garage ceiling insulated with cellulose. Now, the total loop is going to be about 311 ft. I don't feel like injection mixing or 4 way valving my old boiler. I just want to do my thing and let it do its thing until i replace it with something else next summer or it dies a peaceful death Since my mother-in law keeps her apartment about 170 degrees in the winter I was wondering if I could take advantage of that by adding a plate heat exchanger and an indirect tank to supply my radiant floor.
-OR Installing an electric on demand heater closer to my floor. I was leaning toward a combination of both thinking it would save the most money. I live on L.I. where gas is less expensive than electric. I want my mother-in -laws boiler to heat my storage tank without making my indirect a zone for her boiler. I figured I would use the heat anytime she is heating her apt anyway and the odm heater would pick up the slack and combining it with the tank would help with its recovery time. Would a heat exchanger rob to many BTU's possibly causing the boiler to condense? Would I save more money one way over another?