I don't believe that the effects of the Healer's Brooch and the Battle Standard of Healing would stack. The Healer's Brooch effect is triggered when you use a power, and using an item is not the same as using a power. Even if I did allow item powers to count for that purpose, you only use the item on the round that the standard is planted. The rest of the encounter, there is a zone effect around the standard, but no one is using a power. So no, they don't stack.
I agree that the guy allowing multiple Battle Standards of Healing to stack with each other is a wuss.

Zone effects of the same type don't stack, despite the poor wording of the rule. So all you could do with multiple standards would be to increase the area covered by the effect; it wouldn't cause cumulative healing. Each standard is still only triggered when someone in *its* zone heals, granting people within 5 squares of it a point of healing.
As for Healer's Mercy, it is a powerful effect, and could well change the encounter substantially. It's stronger than most other Channel Divinities, many of which are extremely situational. Had it been a daily power, it would have made a nice ace in the hole to pull out when things were looking tough. As an encounter, it's more like a "Get Out of Death Free!" card that can be played every fight. Healer's Mercy has the potential to provide a *lot* of healing once you've stacked your bonuses onto it, although some of it is likely to be wasted due to people being out of range, or not bloodied. Smart players could get some serious mileage out of it.
If you feel it's being overused, the ways to counter it are to try and spread people out, forcing them more than 5 squares away from the cleric if possible. Have a character grabbed and pulled away while others are engaged with foes and can't follow easily. Take advantage of the fact that there are likely to be several bloodied PC's while the cleric is waiting for an optimal time to use Healer's Mercy. Target one of the bloodied PC's with some focus fire and take them out. Hit them with a big area attack while they are all near the cleric and hurt, preferably one that targets enemies only, or to which the bad guys are mostly immune.