1. No, echos of light will tick out its full 6 seconds for every direct healing spell you cast (so it looks like it stacks), try it out. Cast heal on yourself, and watch the tick amount, then cast it again on yourself and watch the tick amount go up. In a Raid setting I tend not to POH the same group twice though just because of the glyph. Take Heroic Halfus for instance, I will PoH 5 different groups, one after another and between that and the other raid healer everyone is pretty much topped of for the next AE raid dmg.
Without looking at logs or seeing how you play I really couldn't tell you what you need to do better. the thing with healing is, that it depends on so many Variables such as
- The tanks skill and gear, ability to know when to pop CDs
- The raids DPS, faster kills = less mana needed
- The raid as a whole ability to avoid avoidable dmg
- You fellow healers oh efficient and skilled they are
- You're ability as a healer, how you gear, how you play, the ability to not over-heal much (if having mana issues)
When ever we are doing progressing fights I look at all these things, I look WOL and look to see how many people are taking avoidable dmg, I look at myself and look at what I am over healing with and how I could improve that. I compare my stats to other priest who are healing the same encounters and see what they are doing.