When moving to an underground site, Bill should impact the site number and then be discarded, but he does not currently. The underground site is played and it's number is modified by Bill the Pony, then when you move to the site Bill is discarded (since he isn't discarded until he is "at the site"). Currently Bill does not modify the site number before he is discarded.
Well, if you follow the rules as they are written, then "when move from [old site]" will not work at all, as the old site is not active, since FP player's token is on next site already.
Bill the Pony will not work as well, due to being discarded, before the tokens for move are added.
I've modified the rules of the game, to make the "when move from [old site]" actually work, so I've ordered the effects as follows:
- play next site if needed,
- playout "when move from [old site]",
- move player marker,
- playout "when fellowship moves",
- playout "when move to [new site]"
- add tokens for move.
Both with the order from Comprehensive Rules and mine,
Bill the Pony will be long gone, before the "add tokens for move" step is reached, therefore it does not influence the number of tokens added.