Putting the plug closer to the incoming intake charge might cause problems with the plug getting cooled off and "doused", especially at idle when the plug is having a harder time keeping hot, or at "idle up", (transition), when sometimes a blast of rich fuel is shot through the transfers if the idle is a touch rich.

I don't see how two (or more) plugs could be used to vary timing. The hotter plug is always going to set off the mixture first....

I think the most productive experiments (with glow plugs) would just be to test various heat range plugs, and see which one a given engine, load, fuel, & atmospheric condition combination likes.... It's going to be different every time one of those variables changes.

AV8TOR