If you follow the vehicle to the edge of the map the amount of resources returned very briefly shows up like an XP counter when your units kill something.
I imagine that you get the resources once the unit safely arrives off the map rather than instantly. If not, this is definitely prone to abuse as well.
Something else to take into consideration is that I don't believe I got any munitions back for recalling my WC-51 that was Vet 2 and upgraded. Just appeared to be 150mp.
I'm not crazy about the idea of upgrading the M3 to a M-21. If I built the M3 in the first place, it was likely because I wanted a rolling reinforcement point or transportation for units (typically units with bazookas). The M-21 on the other hand serves a completely different purpose. If I wanted a M-21 in the first place, I would be really annoyed with having to upgrade the M3 to get it.
At this point, I think the best way forward to make this commander more attractive is outright replacing the two abilities that are real juicy. There is a good foundation here (unlike Armor Company) but it's missing something that rounds it out.
Perhaps if Assault Engineers are buffed to make them worthwhile, they could outright replace one of the current abilities in this doctrine. Two birds with one stone there, a good solid unit for Armor Company and Mechanized Company, and they fit the theme of both of them exceptionally well.
I want to look at Assault Engineers the way I look at I&R Pathfinders, Rangers, Paratroopers, etc. Those are awesome units that add real value to a doctrine. I really think they should be roughly on par with Sturmpioneers considering the cost is the same, their intended purpose is pretty much the same, even the name is similar.
Edit: Just wanted to be clear about what I'm advocating in this post. Remove withdraw and refit from the doctrine entirely. Add Assault Engineers in it's place. Replace Assault Engineers with standard Vehicle Crew in the M3 halftrack. It only makes sense to do it this way so that the M3 ability works more seamlessly, and you get real access to the Assault Engineers that were intended to be in this doctrine from the get go. This of course hinges on the Assault Engineers being tuned to become a truly desirable unit worth the asking price.
Just 2 things -
1. Ass Engies are still not worthwhile right now as other people have currently noted so they require additional tweaking, so putting them in yet another doctrine is downright madness at the moment.
2. I don't know where you based your assumption that these guys belong in vehicle based commanders at all, if you ask me they made a hell of a lot more sense in Ardennes Assault, where the Mechanized company there (Baker) also has Cav Rifles coming in with an M3 Halftrack as their unique infantry unit which anybody who played with them will tell you that they are blatantly superior to the AE, being that they are close quarter combat specialists in the form of Riflemen armed with Thompsons and having different abilities fitted for such cases.