To 1: This is 2 questions. The Trilogy and Triamp are 2 very different beasts. I did not like the Trilogy at all and cannot recommend it. (The clean and crunch are excellent, but the drive channel(s) are like a chainsaw).
The Triamp MKII is an excellent amp, but a beast that is hard to tame. Expect to tinker with it A LOT before getting the sound you want, but it is an extremely
versatile amp with lots of excellent sounds. On the other hand you have to consider that you can get a nearly as good result (of course with less seperated channels) with lots of versatility from a Carvin V3 for 1/3 the price of a Triamp.
Should you lose the Stiletto? Well not for a Trilogy. The Stiletto is not a versatile amp, for the genre it was created it is absolutely amazing though. If you want more versatility you should lose it.
To 2: Yes you can, but not easily. You would need a switchbox in front of the cab and to not ruin the amp which is not connected to the cab you would need something to put an its output. The simplest way I saw this is by just hanging a cheap Behringer 1x12 on the switched output. Then you need also a switcher infront of the amps, to switch you guitar input signal to the correct amp. All in all prolly enough cash to fund a nice selfmade cab to make this happen.
Of course you can do it the hard way: Switch one amp off and plug around, but hardly the solution for a hectic gig...