Your photos of the female mergansers show a Red-breasted Merganser on the left and a Common Merganser on the right. I saw the Red-breasted earlier in the day diving at the edge of the ice at the southern end of the lake where the Mystic River begins. About 1:15 it took off and flew up the Lower Lake, so that’s where you saw it. The most obvious difference is the sharp line between the reddish head and the white neck on the Common, but a more gradual change in color on the Red-breasted. -Also the wispy crest on the Red-breasted, which shows clearly on your photo. Sibley also says the Red-breasted has a thinner bill.
I’m more puzzled by the loons. For a couple weeks, people have been reporting one or two Red-throated Loons on the Lower Lake. Then on Sunday I thought one loon looked much bigger than the other, and one other birder posted it as a Common Loon that day. Yesterday another birder using a 60x scope described the field marks very clearly on his ebird posting - one Red-throated Loon and one Common Loon. I measured the lengths of the bodies along the waterline in your photos and got 36 mm and 30 mm.
I’m inclined to think your photo shows a Common on the left and a Red-throated on the right, but I’m really not sure. Did we have two Red-throated Loons for a while, and now one of each? What do others think? Does anyone have better, closer photos?