Should senior leaders be required to conduct OJT in their respective CMFs to stay proficient?

That is a problem that many leaders forget how to do many skill level 1 task and basic things in there MOS. I think that they should replace the board with a MOS test to help with this problem and do some kind of test as part of higher level promotions.

Yes, why not, it is important for leaders to understand reality!

I would ask for a more definitive definition of "Senior Leader". If we are talking a senior SNCO responsible for supervising the shop or division and advising the CO, I would expect extensive and wide knowledge. If we are talking a CO, I would expect a good grasp of general topics and detailed knowledge of specific problems, i.e. a particular part that has a high failure rate, or a particular procedure or rating that requires extensive time to accomplish or is in short supply. Get to the O-6 or general officer level and my expectations would probably be more oriented toward knowing who to ask the questions that would make the equipment or rating contribute successfully to the accomplishment of the unit mission. As has been discussed in many strings on RP, the success of a senior leader often depends more on his leadership ability and scope and his administrative prowess.