X10's dimmer switches *should* be able to control the lights you want, but you should check the wattage ratings. Locating more than one in the same box created more heat, so you'll want to stay further below the maximum rated wattage for each switch.
AHP cannot get status from X10's one-way switches, so it won't know if you turn the lights on or off manually with the switch. However, a macro to set the lights to a certain configuration would do so regardless of what levels the lights were at before (unless you use relative dimming, which makes no sense in this case).
As for control with your Harmony remote, that all depends on how the Harmony works.
If it can control the PC (specifically running scripts off the PC), then yes, you should be able to control the lights with the Harmony, via the PC (you'll need to install the AHP SDK, and write your own scripts to set the lights the way you want).
As for how long X10 will be around, nobody really knows. There are other technologies out there (Smarthome's Insteon being one that I know at least a little bit about), that would probably also be able to do what you want (albeit with a higher cost for the parts).
I hope that helps.
