Since this is all hypothetical there's no right nor wrong; a strategy like "leading with color spray" can be great or bad depending on the situation.
It can be "ok", or "bad". It's rare that it would be "great" (unless we are talking minions). The foes typically still have options with daze, even if it is just ranged attacks, area attacks, or charging.
When slowed the creatures speed is 2, it can only end it's move 1 square away from it's starting position.
Err, no. If a creature can move 2 with a Slowed charge, it's still 2 squares away from its starting position.
Mythra's low AC is ok, because Mythra has a higher than normal HP and surge count, thanks to that pumped up constitution.
In our current group:
AC 22, hit points 52, surge value 13, surges/day 11
AC 20, hit points 45, surge value 11, surges/day 11
AC 22, hit points 44, surge value 11, surges/day 8
AC 20, hit points 43, surge value 10, surges/day 10
AC 22, hit points 39, surge value 9, surges/day 8
AC 17, hit points 38, surge value 9, surges/day 9 Mythra
Hit point-wise and surge value-wise, Mythra is fine and not too far behind the rest of the team. AC-wise, he's very squishy. He's 4 lower than the average of the rest of the team. That means that he will tend to take 40% more damage from AC targeting (approximately half of all attacks) attacks over the other PCs. That also means that not only will he need more healing and use more surges to get back up to full, the odds of him being knocked unconscious are greater.
It's great that he has so many healing surges for a Wizard and he can once a day go out and have some foes target him to save on other PC's healing surges. Once a day and in a 4th or 5th encounter of the day, fine. Most encounters on round one, that's just forcing the other PCs to adjust their tactics to try to protect him after his suicide run.
The control aspect of a Wizard should hamper the foes, not hamper the action choices of his teammates.
Leading with Color Spray is a sound risk, the rewards are great.
Great? Dazing a few foes?
Sorry, but you're mistaken. Daze doesn't do enough to protect the Wizard. Sure, a Fighter might come along and help, but then again, an undazed foe could always shift away from the Fighter and still often either attack or charge the Wizard.
Let's say that the Wizard manages 3 foes and has a 60% chance to hit.
That's:
21.6% chance to hit all 3, they can all still charge.
43.2% chance to hit 2 and miss 1, 2 can still charge and 1 can do whatever.
28.8% chance to hit 1 and miss 2, 1 can still charge and 2 can do whatever.
6.4% chance to miss all 3, they can all do whatever.
Odds are, he will hit 2 foes most rounds and miss 1, and if he doesn't manage that, odds are that he will hit 0 or 1 targets. 1 encounter in 3, he hardly does anything with this but become a target (it's worse if the foes are higher level). That means that 1 encounter in 3, his teammates are forced to go "oh shoot, I gotta go protect that suicidal wizard, sigh" and are somewhat handcuffed in their own choices.
That also means that he will probably have 2 or 3 foes in his face and one of those foes will often not be dazed and that means that he will need to shift away (or teleport away) from that one foe in order to get away next round. His party will come to his rescue, so the chances of a second burst or blast from him on multiple foes is pretty slim without catching other party members.
And, this doesn't take into account other foes that were not within the blast range, but are now closer to the Wizard due to him moving to the front of the group.
Sorry, but leading with your chin is not the way to box.
Granted, there are some circumstances, like encounter #5 of the day when other PCs are low on healing surges and the Wizard has plenty where the Wizard is merely shifting healing surge resources towards himself in order to save theirs where this could be ok. It could also be good at the end of the encounter or against minions. But as a strategy for most multi-foe encounters in round one, this strategy pretty much sucks.
Note: If Mythra had an AC of 20 or higher like the other PCs, then this strategy would be a lot better. It's the AC that kills this strategy. With 5 lower than the front line types, he's just begging to get pounded on. And yes, he could use his Shield spell to get his AC up that high for that round. But, he's using up a lot of resources in round one, just for the chance to do some damage (the Daze does little) that he could probably do just as well from the back ranks.