As a Linux user i have to advise against Unity. No idea where this 'ease of porting between platforms' myth comes from.
id Tech 4 may be a little more involved, but it's capabilities far exceed pretty much all other open source offerings, plus there is a market open here in that i haven't even heard of any plans for projects to use it since it's source release.
I would say flash it's very easy to learn and would be your best bet imo
The Source engine.
I don't really think that linux should be a chief concern in this situation. I often start to think that when considering a project and then have to stop myself. At the end of the day, it's a *very* small part of the market for 99% of games and as such you shouldn't let support for it rule out potentially easier to work with solutions.
Tech4 is a nice choice and something I'd like to work with but it doesn't sound like it'll be suitable for the people the OP has in mind as they are only 'light' coders and have mostly worked with level editors in the past. I guess Radiant (assuming that's still what T4 uses?) is nice as editors go but it's not really a full on development suite in the same way as the Unity, UDK, etc things are.
Cross platform considerations on a first entry into game development is most likely to detract a lot of effort away from the main product tbh... and if my experience is anything to go by its gonna be hard enough to stay motivated and focused on the core game...
tbh I foresee this being a situation with one talented programmer, maybe 1-2 people with some rudimentary level design or some decent art skills but no practical application of them into a game context and the rest lots of great sounding ideas that have absolutely no hope of becoming a reality in a computer game and after awhile just pettering away to nothing.
Want to post any reasoning? As far as I'm aware Hammer (info may be out of date here...) is pretty nasty to work with. I'm also not sure what the licensing terms are for SE stuff - free mods are fine but selling commercial games may require paying a hefty license fee.
As a Linux user i have to advise against Unity. No idea where this 'ease of porting between platforms' myth comes from.