The correct answers are If I didn't have an early meeting tomorrow, I would stay longer. and If I weren't feeling so exhausted, I would be dancing right now.

Second conditionals can use the continuous aspect in either the if-clause (weren't feeling) or the result clause (would be dancing) to emphasize an ongoing action.

"If I wouldn't be..." is incorrect because would cannot be used in the if-clause.

"If I don't have... I would..." incorrectly mixes the present tense (don't have) with the hypothetical result (would stay).