My default flight for FSX is the  default C172 in a cold and dark setting (Avionics Master Switch is ON). This seems to prevent the throttles being set to full power when I load the 777 (1 in 6/7 starts). The throttles are nearly always in the IDLE position. Pressing "F1" fixes it quickly.

The other developers 737 has an initialization period, upon loading, of 16 seconds where the engines are running. I does this so that nothing adverse will happen when you go to prepare a flight. It also allows you to load it from ANY aircraft you want so that it will flush problems out of your computer system. Then, to select a cold and dark situation, you select it from the saved panel states AND it takes another 16 seconds to make it ready. Those 32 seconds (not including the time it takes to go through the CDU pages to get to the saved panel states) are much longer than it takes me to hit F1 and G (if needed), so I don't consider it to big a problem. That does not mean that I'm saying it isn't a problem, I'm simply saying that it's not a big problem.

The other developers 747 and MD11 have caused my FSx to CTD if I load them after using another complex aircraft. I am always better off starting FSX fresh (with the default C172 cold-n-dark) THEN switching to either of them. Most of, if not all of, the developers of complex aircraft addons recommend starting their planes from the default C172 cold-n-dark setting.

So whenever I switch to any advanced addon aircraft, I switch to the default C172 first. This prevents the CTD for a while, but eventually it will CTD after I have switched aircraft about 5 times.

It sounds like you are starting FSX with the engines already running, so when you load the 777, it moves forward and through the terminal.
I always start it with a cold and dark setting, so when the 777 loads and if the throttles a fully forward I press "F1". It can't move from where it's parked.
If the gear lever is up, I get a warning for that, so pressing "G", fixes it. It's not like the gear will retract and the 777 will drop to the ground. Just a simple warning horn goes off is all.
For me, the 777 does not always have the throttles fully forward when it loads. It only happens sometimes. I guess that because I always start with a cold-n-dark setting, it doesn't cause any problems for me.
If you are not loading the 777 into FSX without the engines running (cold and dark), so that the problem is happening when you start the engines, then perhaps you need to revise your before engine start checklist.