[Estate]

This is a timing question

Running a "Flip Effect" deck with kimono's and rock lee N-571. My opponent has Pakkun N-1376 already in play. During my turn i deploy deidara (younger days) N-1103. I already have n-571 and Dreams of the past M-772 in play. Now i activate N-571 effect flip a coin the results is tails. I indicated that i intend to activate my mission to re-flip the coin. At that time my opponent says "chain" an activate N-1376. I then chain to that with N-1103 and pay the cost. There are no more effects put onto the chain. therefore the chain starts to resolve. I flip a coin the results is tails and for the second time during my turn I indicate i want to activate M-772. My opponent states that since the chain has started to resolve and M-772 is already apart of the chain i can no longer (activate) add it's effects to the chain(even though it can be used twice per turn).
My question is was this the correct answer or am i able to use M-772 to re-flip the coin i flipped for N-1103? Unsure of the answer i went ahead and flipped (as another results of tails wouldn't change anything), the results was heads. Therefore when the chain resolves my opponent's N-1376 who was the target for my N-1103 is injured. Since Pakkun is injured and cant use his effect to negate M-772 i then re-flip for N-571 and the results is heads.