I most definitely believe that people should behave in a gentlemanly manner, meaning that, as you said, I see no problem in someone changing their purchases half way through the combat movement phase (as long as no combat resolution has taken place), placing his units during a turn that his not his, if he forgot, ect… That just makes the game easier for everyone. Also, if we miscounted IPC, we do change the total, both in favour of against a player, EXCEPT for convoy disruptions. If one forgets to remind his enemy not to lose income from convoys, it is his own fault.
Regarding letting people know their mistakes, I think it is fair to warn the other person when he is about to make a clear game-changing mistake.
For example: Thinking that american troops are out of Japan’s reach, the Japanese player leave the Island barely defended.
For smaller mistakes, like the ones you described, players have to live with it. However, if I feel that I have the upper hand in the game, I will tell my opponent, because I don’t want them complaining later.
Furthermore, as I said in another topic, it is common within our games that I player will ask things like “we are all in agreement that no planes can reach these transports?!”, as it is often hard to see absolutely every hidden fighter on the board, without taking 10 minutes of everyone’s time to do so.