Analysis of the effect a opposing voltage source has to a buck-converter's output?
You have to confirm that load1 is really wired that way. Looks weird to me. A more standard circuit would be Load1 in parallel to V1 and the buck. With that it would be much easier to work with, because you know which voltage you have at the input of your buck converter. 

To your second circuit: Having a Voltage source at the output of a buck can cause problems bigtime. Your switching mosfet has a body diode which conducts if the output voltage is bigger than the input. If you build your buck yourself out of discrete components you can ensure that the body diode is capable of conducting that current. If you use a buck IC, that sort of operation is not specified with most parts. To your Question what would happen to the buck: I think it will start to smoke. And if not, its operation will not have an useful measurable effect.

Overall I do not see any use in introducing V2. What do you want to accomplish with this? If you already have a voltage source which you can use for Load2, why designing a buck to drive it from V1? I am lost.

If you would give more information, that may help. What is the use of that circuit? what loads do you use? is this a already existing circuit which you have to expand? is this on a breadboard? ....