The problem is that the Veritas speakers are Energy's (current) top of the line, whereas the EAS line of in-wall speakers are the entry level offerings. I wouldn't want that much of a step down in quality. It would be great if the RC in-walls were around but... they aren't.
Considering you have MultEQ XT to correct minor timbre differences, I would step away from the Energy brand and get better quality in-walls from some other manufacturer. Frankly I don't think the EAS will be that perfect of a timbre match anyway so it's not like you will lose much going to a different brand in terms of "matching", and you will get something with a quality level closer to your front 3 speakers.
I would look at PSB (also a Canadian company, and Paul Barton was schooled in the same principles as Energy's founders) and internet direct brands known for their neutrality and good value like Axiom. There are other good brands like KEF, Tannoy, etc. Basically, I wouldn't worry about timbre matching too much and just get the best in-walls you can afford.
With respect to your three questions:
1. Probably not as discussed above
2. It's just shape, C = in-ceiling (round) and W = in-wall (rectangular). I'm not sure why it works this way, probably aesthetics? But it seems like in-ceilings are always round and in-walls always rectangles....
3. If you are going to be chopping up walls and running wires I would go ahead and get four in-ceilings from the same line as the in-walls you get for surrounds. You don't want to deal with that twice so do it right the first time IMO. Then you know you have a high quality set of surrounds / in-ceilings that will carry with you even if you at some point change up the L/C/R front end.
Also, since I'm spending your money, might as well think about a second sub for a room that large