I haven't done the walk you are thinking about, so can't be a lot of help here! But perhaps I can make a few general observations.
>> 1 day lower Engadine ... I want to wander the villages here and had a couple of ideas, either
>> 1. visit the 4 towns.. Guarda/ (maybe walk to) Ardez/ (train to) Scuol/ (bus to) Ftan, or
It would be hard to do all that in one day. My personal view of the "priority" of towns would be Guarda, Scuol, Ardez, Ftan. But if you want to do a walk, that will determine the priorities for you. Just one thing: don't miss Guarda :-)
>> 2. straight to Ftan by bus, and walk to either Ardez or Guarda, bypassing a visit to Scuol.
Again, I'm not familiar with this walk, except to say that Ftan and Guarda are high on the mountain, and Ardez is down on the railway line. I don't know whether the walk descends to Ardez, or passes above it.
>> The walk looks lovely, and would mean I don't have time for Scuol, even though we would pass through on the way to Ftan.
Passing through Scuol on the way to Ftan you will not see anything of Scuol :) The railway station is on the edge of town, and you can't see anything interesting from there!
>> Would the architecture and character of the other 3 towns (squares, houses, lanes, setting in valley) be similar enough that the walk option be the better one?
Yes and no. The architecture of Guarda and Scuol is very similar, but Guarda is a tiny village, with no through traffic, and is high on the mountain at an altitude of 1653 metres above sea level. I think only residents' cars are allowed, so the streets (lanes really) belong to pedestrians. Views are down into the valley.
Scuol is a larger town and also has modern areas. There is occasional traffic through the lower old town, including buses, and the main street (Stradun) is a busy shopping street with buses and cars. It is built on a hillside, but is basically a valley town with mountain views, although it is a reasonably high altitude valley at 1290 metres above sea level.
Both have great views, but very different. I like Scuol very much, but it has a different character from Guarda due to its size, shops and services, and traffic.
I have only seen Ardez from the train, but would definitely like to wander its streets one day.
Ftan has mainly more modern-looking buildings except for a few pockets of the older Engadiner houses. The short bus trip up there, with views into the valley and across to Tarasp Castle and the Lower Engadin Dolomites is wonderful.
>> I really like the look of Scuol from photos online, but it looks like it would take a chunk of the day to get down to the river and wander those squares in upper/ lower towns.
Yes, I think Scuol would take at least half a day, perhaps even longer, as it is spread out. Guarda is very compact.
>> We will leave Switzerland on the BE route, but regional train, probably stopping en route for lunch in Poschiavo.
Have you considered the restaurant at Alp Grüm railway station as a lunch stop? The view there is superb (photos attached). However, when the weather is poor, the view can be obscured by clouds.