I (southerner) can't advise you, but I can tell you this: My sister in NE Colorado installed electric radiant under 12" tile in her kitchen, and it is truly a joy to walk on! It is controlled with a programmable tstat, so it is only on during high traffic. It heats up quickly, so she can bump it up at other times as needed. Also I think the tstat has a timer, so she can set it and forget it. When she walks on it, she experiences the pleasure and the pride at the same time, so it is a never ending pat on the back.

Agree with Mike above. My house in Kansas City has the air handler in the attic, and a boiler for our main source of heat. We do have a large return in the ceiling of the upstairs hallway, and ductwork running to the main level as well. No AC in the basement. We got a heat pump 2 years ago when our AC needed to be replaced. That is the what I would recommend if you were doing this for yourself staying in the house. Because with the HP, you have fast responsive heat for spring and fall nights when it is in the 50s at night and warmer during the day. In those conditions the house gets cold at night, and then the boiler comes on. It takes a couple of hours until it really feels warm and by then it's morning again and the boiler is off again. Will you electrical panel support the adding of any AC? Something to consider.

Option B will certainly be cheaper because most attics are only built to accept storage items, not the weight of a living area. You'd have to reinforce the attic joists in order to meet structural requirements, and that would intrude into your headroom. Also, even if you did plan from the beginning and installed larger joists, unless you have around 9' of ceiling height, you won't have room for a living space and the required insulation. You'd have a lot more construction complexity with this choice. Option B will also need all of the above mentioned alterations, plus additional insulation in the garage ceiling, but it would undoubtedly be cheaper than option A. However any of these projects will require a lot of work from licenses trades to accomplish. Creating bathrooms close to an existing plumbing stack does help with some of the plumbing costs, but we're talking the difference between 15K and 12K for your plumbing bid. In the overall numbers of the project, 3K isn't a lot of difference on a 100K project in order to have a bathroom where it works best for layout functionality. Option C would be to take the 100K that the project would cost and add that to the amount you could realistically sell your home for and go shopping for an existing home with the additional space you need.

The apparent contradiction of cooling inadequacy vs. heating sufficiency is not a contradiction at all. It is basic physics and it can not be messed with. If you have the heat transfer rates and capacities/contributions of both heating and cooling appliances arithmetic will make sense of it. I specify contribution because you need to know if they are operating up to their rated capacity. Condition of the ducts also contributes, to both heating and cooling as you pointed out. Of course, and the calculations involving existing construction vs. new construction is sometimes challenging due to unknowns in the old stuff. Rather than estimates of heat transfer using assumptions about the construction, you can do better with fuel use rates. You should be able to get real heat contribution from the furnace and fuel use records (and heating degree days) from the coldest weeks of the year when space heating makes hot water and cooking insignificant. You also need to know the efficiency of the furnace. An energy rater or engineer should be able to use that data to calculate the heat transfer rates of the structure. (BTU in must equal BTU out.) Those rates can also be used for heat transfer in the other direction in the summer. Those calculations can be done today. You have fuel use records and you can get degree days from a nearby weather station. It is somewhat less common to use energy use of the cooling appliance for estimates, but it should work just as well. You need to separate electricity use by installing a separate meter and the contribution of humidity removal will have to be factored in. That means that you'd have to record humidity inside and outside and include that in the calculation. It is likely that you have sufficient capacity for you furnace or it is oversized whereas the cooling capacity is not adequate if your cooling unit is functioning properly. Again, calculations will tell you. How do you fix it? Better insulation on the ducts, structure envelope and better sealing of both will always cost less in the long run than installing a larger appliance. Bringing the ducts indoors is also a good idea as pointed out by mike_home. Don't think of active fans if you consider better attic ventilation. It needs to be passive. it is difficult, however, to draw enough air through an attic to make much of a difference in its temperature. Most of the heat transfer into the living space is due to re-radiation of heat from the underside of the roof deck. That is why radiation barriers were so "hot" for a time. The trouble with them is that they get dirty and lose effectiveness pretty quickly. It was a nice idea, but did not work as expected.