If your brother can't do stairs, I would recommend option 2 (entry to the plane is via lift to front right door). Don't know about Thessalonika but I suspect it will be stairs and buses to the terminal.

You don't have to have your own mobility aid - all airports provide chairs and assistance/ Some people bring their own.chairs (those who need the chair for more than just getting to and from the aircraft. At Heathrow, you can opt to self push - when you go to the mobility desk, just let them know that's what you want to do - but in this it is exceptional. Almost every where else there's no self-push option. there will be someone to push the chair for you.

There is no problem getting into the first wing.

As for times, whenever relying on someone else, your timing is not 100% in your control - from when they fetch you from the lounge (if you are not self-pushing) to boarding and in particular disembarking from the plane. Everything WILL take more time, and you should take that into account.

I suspect however that you might be a little less blasé about the "just hobbling ourselves" if you were the one with the torn knee ligament, And then of course there is the question of what happens after you've left the airport in Thessalonica.

Have you considered renting a wheelchair for the duration of your holiday? You can wheel the chair to the gate yourself, and it will be available at the gate on arrival.
It also means that your brother has more freedom to get about (in a pain-free fashion) while in Greece.