" Forced: If Smaug the magnificent is dealt a shadow card with a burgle effect, he makes an additional attack immediately after this one." Does he get a shadow card for that additional attack? If yes, can he get multiple attacks in a row if I keep getting shadow cards with burgle effects? Can I cancel these shadows?

Regarding the burgle mechanic, what happens if when attempting to make a burgle, I reveal a neutral card in the top of my deck and I have to match sphere?

Yes, Smaug gets a new shadow card, so yes he can get multiple attacks in a row. You can cancel the shadows, but cancelling the shadow only cancels the shadow effect, so it doesn't help you with the Burgle chaingun problem. For that you want to discard the shadow cards before you flip them over.

If you need to match sphere of a neutral card, you need a neutral card to match with it.

But neutral is not a sphere!

As I recall, if playing progression style, the only way to stop the Smaug chain gun is to play Dawn Take You All (solo or multi-player), or by sending Rider of the Mark to another player (multi-player only). Both methods can be used to discard a facedown shadow card before it is flipped up.

Unless it has been specifically ruled otherwise, I would have thought that you can't match the sphere of a neutral card with anything, even another neutral card, since neutral cards are defined as having no sphere.