I feel like you stated, the initial fee is too high and that is for a player who has not accomplished anything. If he accomplished two out of the three by the time his contract run out, then that will take him to £51m, which could be more acceptable as he manages to get 25 goals in a season. Dependent on the consistency factor and whether he does activate any of the last two clause, I feel that we still put to much expectation on a teenager who now has a conventional goals that he has to meet or else he will be seen as a bad acquisition. I prefer the more ethical approach of having teenagers play with enthusiasm and joy, while the experience players hold most of the burden.
Your first point is in regard to the expectation that he has to reach such a conventional goal or else, he will be seen as a failure based of price we paid. I do not see how it is a win win for United as putting so much burden on a youngster could fail miserably for United and he could be a expensive mistake if he only gets 0 or 1 of the 3 needed goals.
Your second points has been echoed almost everywhere that, we should not have waited till the end of the transfer window to do such a business with Martial.
http://www.mirror.co.uk/sport/football/transfer-news/gary-neville-calls-transfer-window-6359654
I am unsure about whether the interest with other clubs were genuine or it was a case of lack of preparation by United that made us pay such a high initial fee for a player who has not accomplish much.