If you're using one of the cross glow plug wrenches (the ones with four tips for different sized plugs, of which you only ever use one) you can crank down fairly hard on it. Still, I doubt being slightly loose would keep the engine from firing at all. I've actually started higher compression engines with the plug finger-tight so the starter could turn it over easier.

On your funny glow starter, if those D batteries are in series, they should light up the plug like a torch. Actually I'd expect them to burn it out before too long. If they're parallel I'd guess they'd do it, and if they go through some bit of electronics I've no idea. It's worth borrowing a regular glo starter from someone at the field to see if it helps.

Are you sure you're still drawing fuel? If you hold your thumb over the carb and flip the prop, it should get your thumb wet after a few flips.

Most importantly though: what has your local engine guru said about it? Ears play a huge part in debugging these things.