Just in case anyone else is running into similar issues, I'd recommend connecting Optimate leads for maintaining the battery's health, as you initially planned. Jump starting from a non-running car battery should generally be okay, but make sure to connect the terminals correctly. Running the car positive to the earth on the bike might not be the most practical solution, but it's often recommended to prevent damage to the bike's electrical system.
You can find some comprehensive advice on how to deal with flat batteries and more over at
https://stuck.solutions/.
Would love to hear an update on how things have been going with your bike,
@PurleyKing!