Can't answer your question about the early bus service. If Chip'nDaleExpress (WDW CM) doesn't reply, be sure to ask about it when you check in. AFAIK, early bus service is still occuring.
If you decide to take a cab, ask the cab driver to take you to the Contemporary and then to drop you off near the walking path to the MK. That's the closest you can get to the MK. If the cab takes you to the TTC, you'll have to take the ferry or monorail to the MK entrance. If it is too early for those to be running, you'd havd to take the resort monorail. Be sure to take you PS/ADR confirmation number along. IMHO, it's better to take the cab to the CR and walk than to deal with the ferry/monorail before the parks ar open.
Best wishes -