I don't have much time this morning (got to get the 3-year-old up in a little bit, then head off to work and a 4-hour off-site meeting), but I can offer some brief thoughts. Might be able to provide a bit more later...

First, yes the zone 2 output can be split. You can get splitter cables from places like Radio Shack or MonoPrice pretty readily. The split will produce a few dB of signal drop for both signals, but that can be overcome by turning the zone 2 output up a few dB.

Second, having separate volume control is feasible, but less simple. You have already passed through the pre-amp, where your main point of volume control is located. You can use additional volume controls downstream of the amps, but the ones I'm familiar with will be the equivalent of lighting dimmers: they will reduce only. Thus, if you want the outdoor speakers to be louder, you would need to turn the indoor speakers down. An alternative is to get a separate amp with gain controls for the indoor speakers and turn those gain controls down a bit. (The 7125 does not have gain controls, which is why I'd probably pair it up with the outdoor speakers in that scenario.)

As for the sub, where will it be? Will it be part of the indoor speaker setup? Either way, the issue is the same: there's no bass management at the zone 2 output. As a result, you would need to either have a separate bass management device (such as Outlaw's discontinued ICBM-1, although Paradigm also had a small unit at one time that served this purpose) to split the signals or run the speaker cables from the amp to a sub before going to the speakers. In the second scenario, the sub's crossover would divide the signal and pass on the "high" signal to the speakers while retaining the "low" signal to play itself.

Gotta run now. Hope it helps.