The best level or method for gathering coins

You basically just have to leave it on the portal long enough for it to save, so I think it's just that you turned your system off withuot giving it a chance to save or it glitched. eitherv way you won't be able to get your level-ups back, unless you re-earn them

Legendary/Dark variants of characters have different stats than their "normal" counterparts. Legendary stats start and end a bit higher that normal versions, Dark stats seems to be just distribute differently. Color variants (gold/transparent/glow in the dark, etc.) seem to be the same as regular versions, I'm not sure the color differences reflect in game at all.

Series 2 figures of characters already available for the first games (otherwise known as "re-posed") are identical in stats and usage between games, however, they have extra abilities available in the 2nd game, called Wow Pow upgrades, so if you have a choice between buying a 1st or 2nd series variant, I'd get the 2nd.

Lightcores are compatible with the first game (if the non-Lightcore character was in the first game), although it states on the packaging it isn't. I've tested this personally with Lightcore Drobot (illest of the Skylanders). I couldn't find online if there's stats difference between Lightcore and normal versions, there are Lightcore only abilities, though, so it's possible.

I cannot tell how Skylanders leveled higher than 10 behave when placed back in the first game, I assume they'd still be capped at level 10 and new "Giants only" abilities/hats will be unavailable. This is just my assumption.

Here's a nice handy chart detailing character/item compatibility between the games (like stated before, the Lightcore incompatibility isn't true):