Come-from-Beyond,
so let's assume you have a quantum computer that you can use to mine BTC.
Can you use it to disrupt the mining process or not?
Yes, with a QC you can invalidate last 1000 blocks, generate 20000 empty blocks and stop mining leaving the others with 20-year block times.
Really? Can you back that up with maybe some research?
Last I checked, the only thing that makes quantum computers more efffective at hashing is grover's algorithm, which practically reduces the bit length by half. For the same security, the bit length can just be doubled, so using SHA512 instead of SHA256 on a quantum computer is the same as SHA256 on a classical computer.