In defense of the 3.1 cu ft Frigidaire FL, the controls are manual dials and would not be a challenge for your mother. Virtually all other FLs have electronic control buttons which might be confusing for her. (Having recently taught my octogenarian father how to do laundry, I am thankful he had a Maytag TL with simple dials....). As another reader pointed out, for $2000 you could be TWO pairs and stack them. Two 3.1 machines running at once would surpass any single machine for capacity.
As far as washing a king sized comforter, doubtful in any conventional (center agitator) TL because the space occupied by the agitator. A large (4.4-4.7 cu ft) FL such as Electrolux or LG or the new Frigidaires would do the trick, but I am afraid that even the simplest of electronic controls might frustrate your mother. Why not take her shopping and let her inspect the controls?---that's how many older shoppers buy a tv: they look at the remote control to see if they can handle it.
If you decided to go with two stacked sets, try to find a showroom with a stacked set on display (our Pacific Sales showroom does this) to be sure the dryer height and controls are not too high for your mother. Frigidaire does sell a 3.1 FL washer/dryer attached unit with both washer and dryer control dials mounted below the dryer door; comes in both gas or electric dryer. Though the "list" price for this model is $1200-1300, it's not unusual to see them marked down to $1000
because of newer, "hotter" models on the market. However, if you buy the more usual separate units and stack them, the dryer controls will be on top. I've seen the 3.1 washer on sale for under $500; if your water district or utility company is offering rebates, it's even less (you generally won't get rebates on a TL because they use at least twice as much water).
I have a neighbor down the street who bought the combination Frigidaire unit because she wanted dial controls and would have had trouble reaching controls on the top front face of the dryer. In her case, of course, the downside is that if one component (washer or dryer) cannot be repaired, she has to throw out the whole unit.
Question (forgive me if you addressed this elsewhere): are you planning to keep your existing dryer? In other words, is all this advice about stacked units not applicable to your situation? Given your need to conserve water, you might want to take your mother to look at say Electrolux base models, which are now down to $1000 and 4.7 cu ft (enough to do a comforter) to see if she can handle the controls. A touchpad is out of the question, but maybe she can handle dials and buttons. Some of these washers also have programmable "custom" programs where you could create a one-touch procedure for your mother to use. Bear in mind that since the final spin speed on FLs is faster than on TLs, the clothes come out much dryer and drying times are about halved. In my case, I have a Frig 2140 (basic 3.5 cu ft FL) with a spin speed of only 950 rpm (not that impressive). Perm press clothes dry in about 25 minutes, including cool down, and heavy loads like towels are dry in about 45 minutes. With my former TL, the drying times were twice as long.
http://www.frigidaire.com/products/home-appliances/washer-dryer-combination
