What is needed is a total overhaul of the approach, tying more rewards to the activities of the guild as a guild and not the individual contributor. Guild mats, etc. should be tied into guild achievements so that every time your guild completes and instance as a guild for example they would get guild mats as a reward.
I don't think this is a good idea. What if your guild loses many players (as most guilds already have) and can't do much? This just creates another symptom of the same problem of so many people having quit, and it won't solve anything.
I don't understand your rationale. If players gain more guild mats, etc. based on guild activity then they gain more when the guild gains more. As it is now, the guild gains nothing no matter what achievements are accomplished and players either achieve marginal gains from showing up to siege or have to grind a lot to get anywhere. In other words, there is really everything to gain and nothing to lose.
As for what it solves I would think it is self-evident. Guilds that are rewarded by doing more things together will seek to do more things together. If they find they don't have the people they will see an advantage to recruiting more people. Smaller guilds might even start to consider merging to become larger guilds. Larger guilds will have the people to at least attempt to run more instances, etc, etc.
I realize many RoM players have a love affair with creating very small guilds and keeping to themselves but maybe a few less 200 point siege guilds and a couple more 2k ones might shake things up a bit and help the game survive long enough to see the server mergers?