"A lost cause, that is."

Sure, they're correct. The appended "I say" tends to sound a bit haughty or archaic, while the "that is" sounds a bit colloquial, but both are well within the bounds of accepted usage.

That is probably not a good habit. It certainly will make you sound more "foreign."

Such phrases aren't confusing, but they don't seem to express anything. Few people nowadays add such flourishes; common ones I can think of are man, dude, yo, or no? — all of which relate to some kind of stereotypical, hackneyed speech.

By the way, the idiom is lost cause, not lost case.

Of object and subject just an inversion it is... Kind of natural, it's easily understood, thats why. Worst contortions have we heard from all scopes of combinatory spoken. Etc!