I don't want to organize miners to force people to use p2pool.
More miners would equal smaller per-block payouts, but more blocks - so it evens out.
Huh?
It would help the problem of centralization because, as you must know, p2pool is not centralized.
Bitcoin isn't centralized either, but that doesn't mean there's no centralization problem in Bitcoin.
1) I'm glad you don't want to force people to use p2pool.
2) You said "Among other things, block rewards would only be a bit over 5% of what they would be otherwise." I can only interpret that to mean that because there are more miners the reward per-block would be smaller, which is true - but if there is more hashpower on the pool, the pool will also mine more blocks. These two changes are inversely proportional, so they offset. (i.e. 0.1 btc per block * 5 blocks per day = 0.5 btc per day. 10 times as many miners means 0.01 btc per block payout, but 10 times as many blocks - which is 0.5 btc per day - the same as before.) If this is not what you meant, then I have no idea what you mean.
3) Again, I have no idea what the point is that you're trying to make with this statement about bitcoin centralization - whatever it is, it has nothing to do with pools in general, or p2pool in particular. You seem to have strayed a long way from your initial question, and from the topic of this thread.