1. Yes, you can set one AP as Root-Bridge and the other one(s) as Nonroot-Bridge.
2. The 1242AG has 2 antenna ports for the 2.4GHz Band and 2 for the 5GHz Band.
I would suggest you to use the 5GHz Band, because there are several interferences in the 2.4 GHz-spectrum like microwaves or bluetooth. And the 1242 uses DFS (Dynamic Frequency Selection) in the 5GHz-spectrum. This means, he checks for channels which are not used by other APs befor using them by himself. This avoids interferences with other Access points, especially in outdoor areas.
Oh, and you only have to use one antenna. We used the AIR-ANT5195P-R for a similar installation like yours, and it works for distances of 150m and more.