A. "channel update" is a message from one peer to the other about the variables (dust, limit, fees) its not a edit or signing session of a commit, its not a edit of a output inside a commit.

B. atleast you are now admitting these messages are in LN msat denomination and not a commitment sat denomination (1 step forward for you)

C. alice wanting to pay eric 1000, would know bobs ''fee_base_msat". but she does not know carols or diana's becasue she has yet to gossip via the hops/nodes.

D. because of the limited knowledge of (C) alice cant commit to 1003 to ensure bobs 1fee, carols 1fee and dianas 1fee is met to ensure eric gets his 1000, becasue alice does not know carol or dianas fee yet.

and these messages to find out this stuff, is all node and route gossip and channel gossip.. not actual changes to commitments