You would have a higher-leveled chansey, unless it had 200+ happiness, in which case you would have a higher-leveled blissey. If you were talking about happiny, which evolves while holding an oval stone during the day, then you would have a higher-leveled happiny.
1. There's a reason that Hail is used as much as the other weathers, Ice is a horrible defensive typing.
2. For Thundurus, it really lacks true counters, but if it lack Grass Knot, Unaware Quagsire can take it, if it lacks Nasty Plot+Focus Blast or Hammer Arm, Blissey/Chansey can take it, and if you can bring it in on a revenge kill or a Thunderbolt, Banded Mamoswine's Ice Shard could probably KO (Correct me if I'm wrong, I don't feel like calculating it). Also, Froslass outspeeds, and can KO with Ice Beam/Blizzard, or paralyze with T-wave
3. For Haxorus, not much can take hits from it, but it only has 97 base speed, so you can try and find something to outspeed and KO on the revenge, or use Wobbuffet and just Counter it.