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My questions are:
1. If all my opponents characters have already been tapped, can I still attack with the newly formed Combiner?


If all of your opponents characters have already been tapped and you finish an attack with your untapped Combiner, you will then attack a second time with the Combiner.

When Bolster activates during my opponents turn, and Multi-Mission Gear is played with a Combiner Enigma, where does the attack land? As the defending character is no longer there, does it hit the Combiner?

The character they were attacking is part of your newly formed Combiner, so they are now attacking your Combiner.
Source: Wizards of the Coast (2019-08-23)    【Link to Ruling】
Tags: Combiner