Where, as in most places, it's also used for regular bus services, there can be no question of treating it as a platform where possession of a valid ticket is necessary.
Absolutely not. Since this thread is all about the Huddersfield closure, let's take Slaithwaite as an example. Journey times are already typically more than doubled, with no reduction in fares to reflect the extended journey times. It would be totally unreasonable to expect passengers also to walk 10 minutes up a steep hill, and back again, in order to purchase a ticket from a machine on one of the platforms.
Not entirely. Slaithwaite, Marsden, Deighton and for part of the closure period several stations on the Penistone line.
I would have no problem with ticket checks on replacement buses, provided ticket purchasing facilities were made available at the boarding location.