1) It says "all spells become visible" so I take that as meaning visible to everyone. Otherwise it would say something like "all spells become visible to the caster". There is the parenthetical option if the GM doesn't want it to be visible to everyone. This is definitely something that is subject to how you want your world to work.
It doesn't say anything about light but I agree with you, I imagine it as luminous. If you are using Power Perception instead of making it visible, I imagine Power Perception can be used regardless of lighting conditions, although again this is a GM call.
2) It's kind of borderline. My personal feeling is that Eldritch Weapon overloads the item's aura with raw mana and would burn out the bladerunes, or the bladerune enchantment ("Bladerunes are short-lived enchantments placed upon the exterior of an item's aura") would interfere with the Eldritch Weapon spell. But that said, I don't see a significant balance issue with allowing it.
3) It's a utility spell so by definition it will automatically fail to affect any unwilling target. I was initially thinking the spell would end (at least for the individual now in conflict), allowing them to retain their remaining PP as if the spell had ended, but actually I think it would be more interesting to just let it continue as the pool is already established. You could allow the caster to end the spell prematurely.
I don't read it as saying the participating spell users have no PP of their own. Rather, their PP is being freely shared. But it would be reasonable to say the capacity of the pool is not exceeded unless the total PP available exceeds the total capacity of everyone present -- overloading a single individual may not cause a problem if someone else is below their capacity. If burnout occurs, I would roll for one target at a time, going in arbitrary order (if it was triggered by someone getting hit with a Mana critical start with them), until the pool is below its capacity, so that could affect everyone but might not. Likewise, Power Drain is possible only if the total combined pool is emptied. You could rule that having no power left at all immediately ends the Mana Pool spell, since there is nothing to maintain the pool.
I haven't encountered these in play so take that as off the cuff remarks rather than experience....