Hi, Eamann, I have walked both, so I'll offer some thoughts in response to your post.
CDM -- It does have a clear ending point in Sahagún where it joins with the Francés. If you start at the Church of Santiago and San Juan Bautista in central Old Madrid, you can walk the first day to Tres Cantos, pretty doable for most I think. The walk is quite nice through central Madrid, up the leafy Paseo de la Castellana, past the Bernabeu stadium, though some tony residential areas. At the Plaza de Castilla, the arrows begin. It is NOT an industrial route out of the city by any means, in fact it is one of the most pleasant routes out of a big city that I've had on any camino. (Think going out of Burgos rather than going into Burgos). As soon as you cross under the M-30 (or maybe 40, my memory is not clear) you are out of the city, and start walking through open land. Infrastructure is great, pilgrims VERY VERY few. I don't think either of these routes has much prospect of company for you, but you may be lucky and find someone.
Cami Catalá from Montserrat. The part up to Huesca is, IMO, not terribly interesting or attractive, though I did have a great time because I was walking with LTfit. We met a few others after Huesca, and that's when the scenery really gets nice. The Catalá actually merges with the Aragonés at Santa Cilia de Jaca, not at Somport, but LT and I hitched a ride back up to Somport so we could walk the Aragonés from the border to Puente la Reina. I love the Aragonés. Starting in Somoport and walking "backwards" from the turnoff at Santa Cilia to Montserrat would be hard, IMO, unless you have a GPS.
You can see my blogs if you click on the links in my signature below for the places you are interested in.
I also have posts on both of these routes with more in the way of practical info-- distances, lodging, etc.
https://www.caminodesantiago.me/com...catalán-from-montserrat-through-huesca.36443/
https://www.caminodesantiago.me/community/threads/my-leisurely-stages-on-the-camino-de-madrid.11148/
Other forum members also have long detailed posts on the CdM, but I have mine bookmarked so I don't have to search. If you go to the subforums and scroll through, you´ll find a lot! Buen camino, Laurie
p.s. I'm going to merge the two threads, because having two identical posts going always just confuses people. What I would suggest is that we could leave it here in Camino de Madrid for a while, and then after some time we could move it to Camino de Madrid, but I honestly think that most people will see this no matter which forum it is posted in.