Any of the touring snow shoes should be fine for your parents. I would avoid the MSR style mono-frame shoes, as in my experience those are neither as easy or enjoyable to use. As for weight, most manufacturers have a maximum recommended weight, but it's really a balance between usability and packability. A larger snow shoe (within reason, of course) will always be better for on-trail touring where you don't have tight turns between trees and such. My personal favorite is the Atlas 12 series, which I have the 30 inch version of. I've never had a problem with flotation with them, even when my combined weight (body + pack) is pushing over 200 pounds.

You might be able to strap them on, but they're going to be incredibly stiff and awkward to use. They're intended to be used with some kind of winter walking-around boot.Downhill boots are incredibly uncomfortable to use with snowshoes. AT setups are rather cheap now, do that.

I'd go with the Redfeathers from personal experience. Here's my earlier review:"Good flotation on all but the freshest, deepest powder. Great on trails & packed powder. Easy to adjust claw binding. Very durable. V-tail allows for easier, more natural stride. Remember to add in pack weight when selecting size."