> who else has or is planning to develop a methane engine?
BO's New Glenn first stage, RocketLab's Neutron, and Relativity Space's Terran 1/Terran R, are all/will all be methane.
> which other widely used engines used H2?
Ariane 5 and 6, and the upper stages of previous Ariane rockets
https://en.wikipedia.org/wiki/Rocket_propellant#Current_cryo...
Edit: Long March 9 will be mostly methane, with hydrogen upper stage.