If you don't remember from previous megas, you can get the stone before obtaining the Pokemon and that will enable you to use the Mega as soon as you catch the base.
To your second point, Meowth coin farming is not difficult at all. You should be able to generate in between 5k to 10k a day depending on hard the skyfall RNG screws you over. (If you're playing on Mobile, then I suppose I could understand the statement, but playing on mobile is stupid in the first place)
I already farm Meowth. It just isn't enough, which is why I'm so far behind in the main stages. I suppose that I
could spend the necessary coins, and store the Abomasnowite, but then again, it'd probably cost me less in the long run if I just wait for the non-timed stage, rather than fighting tooth and nail against every other John and Joan who wants one of the damned things themselves, especially because I hate timed stages, not being so good under pressure.
I suppose
could play once every 2 and half hours, but Arceus help me if I'm too busy with something, or my dad is in a bad mood, and catches me playing.
And I haven't reached Roserade yet (just beat that stupid Roselia stage), but something tells me I'm going to need a bunch of coins for that. Definitely not getting Abomasnow anytime soon.