Sometimes even professionals do the dumb. 5e makes this worse by encouraging quantum actions the GM is forced to adjudicate rather than giving both sides clear actions & struyctures to be leveraged.
No.. that's just wrong. The players only get so much leeway before they are disrupting the game with retcons. In this too it doesn't matter if Bob was paying attention & "missed it" or not. That bolded bit is not a unidirectional thing as you are making it out to.
As a GM I've
already done that when I said "the orc is going to attack Bob three times". If Bob wants to use his reaction the time to do so is immediately & fast before I
start resolving those three rolls rather than after I've resolved the telegraphed attack roll for shield/parry or the attack roll and damage roll for uncanny dodge. Unfortunately the way these abilities are designed is one that pressures the player to not pay attention & retcon for maximum effectiveness so I can't even houserule that the reaction must be taken before the rolls are made without directly nerfing my players &
casting player resentment. Sometimes badly designed abilities are just objectively bad for gameplay on too many levels to justify any good ends that comes from the means of their mechanics.