Nope
The Nest can only control furnaces that use "on-off", 24VAC control systems, such as conventional and two stage furnaces. Fully modulating communicating systems (such as your Lennox iComfort, or the Carrier Infinity) use proprietary serial protocols that the Nest doesn't know how to speak.
This means that in order to get the full benefits of modulating operation at this time, you need the matching system thermostat -- otherwise, your furnace is simply a rather good single or two stage furnace, not a modulating furnace.
The wire currently hooked to Y and C will be going off to your air conditioner condenser unit. Y is cooling.
C is common, and yes, you can connect another wire to that terminal.
You appear to have a two-stage furnace however, with W1 and W2. That means you have a couple options:
1 - Install another wire
To take full advantage of two-stage, you need to connect all 6 wires (R, C, Y, G, W1, W2).
You can run another physical wire, or buy 3rd party accessories that add a unit at each end that allows doubling up of some of the wires.
2 - Run as single-stage
You can possibly run your furnace with only W1. Many (all?) furnaces will automatically go to the second stage after they're run in single stage for a few minutes. This isn't as efficient as if your thermostat tells it to immediately go to second stage when needed, but is still usable.
3 - Run without fan control
You can potentially skip hooking up G (fan) but this means you can't run the fan in 'on' mode (or use periodic air circulation features) -- it'll only run when the furnace or A/C is on. (I'm not entirely certain about air conditioning with this setup though -- I am fairly sure A/C call will turn on the fan even without G connected, but not 100%, and it may vary by furnace model -- be sure to test if you are going to do this).
Best Answer
Add it to the wirenut with the fat blue wire in it
On a typical split-system with air conditioning, there will be a two-wire cable running out to the air conditioner with one wire connected to the yellow Y wire and the other wire connected to C; this cable carries the call-for-cooling signal from the indoor unit to the compressor unit's contactor.
As a result of this, we start our trace at the Y wire, and trace the wire connected to it back to the outdoor unit's control cable. From there, we can deduce that the other wire must be connected to C in order for the air conditioning to work, so we can simply add the blue C wire going to the thermostat to that wirenut. This is confirmed by the fact the wiring diagram you posted shows that connections to the secondary-side ground (C) are all blue wires, and the wire nut in question has a fat blue wire going into it.