Well, actually your questions are being broken down into several somewhat relevant but not really wholly-related questions.

For the first question, no, quantum computing is not a real threat to cryptocurrency for the simple fact of the electricity cost.

Currently, in order be a real threat to cryptocurrency, one would need to accomplish the attack known as '51% attack'. Literally, from the coined term, we could know that the attacker need to control the simple majority (>50%) in order to launch an effective attack with interfere with the underlying blockchain. They can theoretically monopolize the creation of new blocks, which allow them to exploit the vulnerability of double-spending. But fortunately, this would need a large amount of computing power as well as electricity. 

While quantum computer could possibly attain high computing power, and hence pose threats towards cryptocurrency. Let's not forget that quantum computer is made from specialized design consisting of superconductors (They need to be cooled to a outrageous level of -273°C, to function properly!). This would use a lot of electricity, hence render this threat currently impossible, at least from a technical perspective. 

For the rest of the questions, they are more about the computational ability of the quantum computers, which the answer would be simply yes! But still, we still got a long way to go, and currently the biggest threat for cryptocurrency would be scalability. This deserves more attention, IMO.