If this was a controller based solution, and you wanted to do something like a "repeater" mode, I'd tell you to turn them all into Mesh APs, and use the A radio as backhaul and the G Radio for client access. I think this is much better than just repeater mode....
However, since you aren't using a controller, I wonder if you can set up bridges with the A Radio? It wasn't an option in LWAPP to bridge the A Radio and Access with the G radio, but the mesh has allowed that. Maybe it can be done with autonomous? Anyone know?
Instead of running cable, have you ever considered Ethernet over Power? (not Power over Ethernet)....
The concept of sending your ethernet connection across power lines? Just a thought, I haven't ever implemented it myself but I've played around with the devices and they appear to work. Don't know about throughput though.
The two antennas (potentially four, depending on the AP, two for 2.4GHz 802.11g/b, two -or a paddle- for 5.6GHz 802.11a) are for "diversity."
Diversity is a way to enhance the likelihood that the signal is sent and received in good shape (reduces the effects of "nulls" and multipath). The bottom line is that only one antenna is active at a time (for each band).
As others have mentioned, if you had the dual-band version of the AP, you could serve clients on one band, and use the other as a communications link between the APs ... the hook being that you must have good signal quality on the inter-AP link.
Cabling, if it is at all possible, is your best bet. A good site survey would likely be your best resource to make that kind of decision. Buying the stuff, then finding out it won't work kinda sucks.