It is hard to say if 12 mm/sec is too fast. You will need to do some prints and see if you get any 'layer shift'. You might be able to turn up the power to your stepper motors and if so, that should let you run faster. It is OK for the stepper motors to be so warm (hot) that you can only hold your hand on them for 5 or 10 seconds.
For checking out if your endstops are working... You can use M119.
As far as bed leveling... I can answer any UBL question, but there are 5 different bed leveling systems supported by Marlin. And the answers are going to vary depending on which one you are using. (And you didn't specify this.)
For checking out if your endstops are working... You can use M119.
As far as bed leveling... I can answer any UBL question, but there are 5 different bed leveling systems supported by Marlin. And the answers are going to vary depending on which one you are using. (And you didn't specify this.)