In the Railcave, if you head left first right away and proceed to take every door marked by two stones, it should lead you to a room with one of those unopenable doors, the TM for Trick Room, and two shiny rocks to mine ^-^
As for missing out on Ponyta, on my first playthrough of the event, I somehow glitched it up and I didn't get the Ponyta as well. The gang says to meet them in that abandoned house at night, and when you do, that's when they make up the plans to steal the old lady's rare Pokemon.
Just another thing to add: I noticed in the Pokedex that Houndour is catchable in the wild, possibly in the Team Magma hideout, although I've yet to find one.