Quoted:
I would go with the shotgun and choose my loads to make it manageable. In real use you won't notice recoil as much but the reduced recoil loads do help you get back on target faster, particularly if you're not in a nice steady shooting stance.
Outside the listed options, an SKS would be my choice for cheap effective firepower. You get a little reach, you get rate of fire, you get durable and you get inexpensive. Heck, you also usually get a bayonet, just in case.
The folding AK's would be nice, but in my state they're a no no so I haven't paid much attention to them. Something like that, with a few mags, makes a big difference in effective firepower in a compact package. Stick it in a gym bag and tuck it somewhere a thief isn't likely to look if they're in a hurry.
One problem with AKs is that they are the weapon the bad guys use. Does any US PD use them?
The SKS looks kinda like a "normal" fudd gun, at least without the bayonet and extended mags, but they have been used in lots of police shootings and might be recongnized by the local PD as bad guy guns as much or more so then AKs.
I'm not saying AK and SKSs shouldn't be used, as you point out they are very well suited to the task of trunk gun in many ways. I am saying that you need to consider the optics of how you look.