First, Happy Belated Birthday!
Second, I loved your enthusiasm for mouseover macros!
I agree that there are times it can get a bit finicky but I still agree that they are made of awesome.
I threw up a template of the one I use on WoWWiki. You may find it a handy too. I'd love to see what works for you too cause it's probably much better than my humble attempt.
/cast [mod:alt,@player][@mouseover,help,nodead][@mouseovertarget,help,nodead][help,nodead][@targettarget,help,nodead][@player] Flash Heal
Basically since the first conditional test in [ ] to test true is the one that's used, you can re-order the tests to suit your play style. If you want an actively targeted, live, friendly player to get your heal before the mouseover target, just move [help, nodead] so that it is earlier than the mouseover stuff.
The way I have it in the above example, you can self-cast with the alt modifier, otherwise it will cast your spell on the first valid target in the list:
1. The unit under your mouse pointer (aka mouseover) if friendly and alive,
2. Mouseover's target if their target is friendly and alive,
3. Your target if friendly and alive,
4. Your target's target if their target is friendly and alive,
You could take out the [@player] at the end and replace it with [ ] if you don't like the self-cast at the end and have auto-self-cast turned off in your interface options. instead you'd get the blue pointy finger that you can use to select the target of your heal.
I haven't played a holy paladin but I imagine for those healadins that like to melee the targettarget part could be quite helpful for main tank healing. You can whack away at the boss and if you cast your heal with the boss targeted, your heal will land on the boss's current target (without dropping your current target). Likewise, if you're doing main tank healing, just mousing over the boss will cause the heal to land on the boss' target. I could see uses for it in PVP as well.
You can test it out in Dalaran. Find a member of the opposite faction that is targeting someone from your side and try actively targeting them and casting. Then try just mousing over them and casting... your heal will land on their target. If they stop targeting your friend, it'll self-cast. If you mouse over or actively target a friend, they get the heal.
I'm sure it does too much for most people, but I like it.