He is playing as the #1 centre for the same reason that Burrows worked well for the sedins.
Sometimes it takes a hard nosed player that can take the punishment and retrieve pucks to benefit the top line. Is he an elite top 6 forward, no. Is he going to be a ppg game breaking centre, no.
Top lines are 100% better 100% of the time with possession of the puck, that is Danault's job, get the puck to his line mates to create something out of nothing. He is doing a fine job.
Is this a long term plan for MTL, probably not. It's a temporary gig, that seems to be beneficial for everyone so far. This is great for Danaults growth and development moving forward. Who knows he could very well be a Mike Richards in the making.
Compatibility wise similar situations have happened plenty times before with the likes of Zajac, Langkow, Handzeus, Legwand, Bozak, Rucchin, Morrisson etc.. all playing top line minutes and having a rather successful time in doing so.
Ideally everyone wants their #1 to be an offensive weapon, Galchenyuk wasn't making it happen so they are trying alternative measures. The Habs are now one of the hottest teams heading into the playoffs, coincidence? Could be.