I think it would be fine to duct through the crawl space to the outside for your air supply. Back when I worked for a living, I spent a considerable amount of time laying down vapor barriers and insulating interior foundation walls and sealing foundation vents. I considered the crawl space to be an unconditioned portion of the interior of the house.
You said in your first post that the crawl space would have “temperature and humidity controlled vents”. I assumed those vents were in the foundation wall between the crawlspace interior and the outside. I have very little experience with ICF construction. Most of my work was on 20 to 120 year old wood framed houses. Tell me more about these vents you mentioned. I think the crawlspace should be sealed airtight from the exterior and thus the need to duct through the foundation wall for outside air.
I’d rather not get off topic here but I’ll answer what reasons moved ME towards a crawlspace. Multiple reasons. Easy install and access to utilities being a big one. Also We’re on a slope and The crawlspace area is not unusable. Once I line it, It’s a great place for storage of kayaks, bikes, Christmas decorations, large tools that are seldom used, etc. etc. We had this in our last house and it proved quite handy for storage, to repair busted water lines, run extra data lines, etc.
Construction is done differently in different parts of the country. In my area (Portland Oregon) slab on grade construction is considered undesirable. Your question is sort of like asking why use trusses that form an attic when you can build a flat roof. Local codes and design styles vary. As a direct answer, many reasons. Future alterations/repairs are way easier if you have a pathway to run wiring, plumbing, ventilation and etc. A huge issue in many areas is also radon. People die from it and slab on grade is a primary way for it to enter your house. Yes, you can sometimes control it with subsurface ventilation, but it is far more controllable with a crawlspace than it is with a slab.
Definitely no. Those exhaust fans would work against the stove trying to pull air from the crawlspace. It's keeping negative pressure in that area which would be dangerous if it reversed draft on the stove. The stove's OAK should be ducted to the outdoors through the wall, either in the crawlspace or at the stove level.
It is usually 3" sometimes 4" so I would put a 5" sleeve through the forms
