Questions 1 and 2, short answer yes, the reality is no. Keep in mind most systems rated efficiency is at a 50 degree entering water temperature, so if your loop temperatures drop to 35 degrees(fairly common) not only will your heating capacity diminish but so will your efficiency. You also have to account for costs to pump the water which if i remember correctly are not part of the COP number you see at rated but I could be wrong on this. Also keep in mind that rated efficiency depends on what stage the heat pump is in, sometimes it can be a 4.8COP in first stage but only a 3.8 COP in second stage so there is a lot more to take in than rated numbers for a system, but most two stage systems are fairly close in efficiency except for the WaterFurnace 7 Series that is up to 30% more efficient than other top of the line systems(5.3COP 41 EER at part load)
Question 3 If your heat loss is 55,000 BTU's this is only the heat loss on the design day(usually coldest average day of the year or a 1% design load) so if on the coldest day of the year your house needs 55,000 BTU's then 50,000 BTU's is heated by geothermal and the additional 5,000 BTU's is heated by strip heat. Of course most days it is warmer than the coldest day of the year so lets say your design temp is 10 degrees outdoor temperature and the house needs 55,000 BTU's when its 17 degrees out your house may only need 50,000 BTU's, this is called your balance point, the temperature at which your house does not need backup heat so in theory you only use backup heat about 1% of the time not 10% of the time.
If you are looking for costs a professional in your area should be able to model your costs based on your heat load based on your specific efficiency.
What area of the country are you in. Are you doing an open loop or closed loop geothermal system?