1) The BM only applies to the person it's applied to, just like exp. boosting shoes.
2) That one I'm not sure about, have never used BMs while leveling up a guild, will have to get someone else to answer that.
3) I assume you mean if both the leecher and leech-ee(sp?) use BMs would the exp be 2x instead of 1.5x? If so, the answer is no. The leecher will receive 1.5x exp and the leech-ee will receive they're own 1.5x bonus exp.