The thing I like about M Glalie EX is the fact that there are multiple ways to set up his attack for the extra damage, from playing Faded Town to Ariados to something like Hard Charm to ensure he survives OHKOs from things that try to set up for them. Personally, I like playing him with Ariados because it allows you to A. Slowly set up to 10 counters, which can be controlled alongside Rough Seas (remember that Rough Seas is an activation, not mandatory!) and B. Poisons your Opponent's Pokemon most (note: most) times, allowing you to OHKO much easier.
As far as Lapras goes... you really do not need her as more than a 1-of. Also, I would personally use Delta Articuno, Regice, or something with a low Energy Cost as a secondary attacker. This allows you to set up your M Glalie EX's a lot safer, which allows you to also set up Cryo Mouth plays if you decide to go with Faded Town over Ariados + Rough Seas. With that in mind, M Glalie EX is not that slow to set up, but is not an easy concept to work with, either. You have to be very careful with how you set it up if you decide to use Faded Town since they can simply Lysandre a benched M Glalie EX for an easy Knock Out. It really is easier to use Ariados + Rough Seas, but keep in mind that you can only set up one M Glalie EX at a time (which may not be an issue overall) and it is a lot slower of a process in most cases.
Still, I believe M Glalie EX is very viable. It will take more thinking, but it should be a strong deck IMO.