Unable to engage Maintenance mode

Why do you need maintenance mode, to check for leaks? If so, I would fugedaboutit, just make sure all drain bolts are torqued, you've put in the proscribed quantities, done.

If you're still wanting to do it: are you pressing the brake pedal when shifting out of Park, that's a requirement, and often not noted in the instruction.

 

Probably just to ensure all loops are full.

Run it for short cycles and check the levels. Add as needed.

 

Regarding the engine coolant change, I suggest you set the cabin temp to MAX HEAT, turn on the fan to high speed, rev up the engine to 3,000 RPM and make sure that the cabin heater produces heat and does not produce the sound of air bubbling in the heater core. If the heater does not produce heat or you hear air bubbling, keep revving the engine for 10-15 minutes or until the air bubbling stops. Make sure the radiator and the overflow reservoir is full, after the engine cools down.

If you drained the coolant heat recovery system canister, I hope that you ran the CHRS pump manually several times, to help fill the canister.

 

Probably not mentioned because it goes without saying that to shift out of park to another gear, you must have the brake pedal pressed. If one blindly follows instructions without thinking (and I include myself in this), it would be helpful for a note to remind one to have their foot on the brake at that step.