M.atk has absolutely no effect on your buffs. It affects heals, but not by much. It also affects the landing rate of your root/sleep/wind shackle (but not the duration/effect).
1. no, buffs have no power.
2. nothing else to my knowledge.
3. probably but cant recall where i got the info from. about heals: there was a table w/ different weapons/mattack and the results leaned to the conclusion i posted.
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edit: prayer buff (bishop only) and curse disease (necro) debuff affect heals. it's percentage increase/decrease (not perfectly sure, though)
1. Does the same (MAttack square root added to base power) is true for buffs as well?
No, it is not added to buffs.
2. What else (if any) is added to the base power of the heal?
Spiritshots will improve your heals, and Bishops get a buff that also helps healers heal for more. That's really all.
3. Is there any online document with all those algorithms?
Yes, but I don't know it because seeing and/or knowing the algorithm is not necessary. Regardless of exact numbers, the effect m.atk has on healing is negligible at best.
There's a thread on here somewhere were we discussed this to death already. I documented it but basically the weapon has almost no effect on healer, empower add a tiny bit, prayer adds more depending on level. And of course sps and bsps add a good amount.
Mind that BSpS, Empower and the weapon very likely work on the same formula, and just have a noticably larger bump up than any of the other factors: Empower adds 60% to MAtk, a weapon is difficult to capture in a percentage, +INT would add a very measly amount to MAtk (I don't even remember the number, but it's a bit lower than Emp), and blessed shots add 300%. That sort of does put everything else in the dust.
Here is (http://boards.lineage2.com/showflat.php?Cat=&Board=questions&Number=615107&Fo rum=f31&Words=matk&Searchpage=0&Limit=25&Main=6150 36&Search=true&where=bodysub&Name=217&daterange=1& newerval=1&newertype=y&olderval=&oldertype=&bodypr ev=#Post615107) the thread where we argued about matk on heals.
Can you suggest what is the best M Attack Weapons to be using at CG & BG?
Forget matck, it has no bearing. For C grade a +0 humonculus with acumen will give a speed boost for quicker heals. For B grade the Sword of Valhalla with acumen also gives a speed boost, and uses less shot per cast.