1- why does mashing fruits require both a) shinui and b) miyad?
Even according to the opinions that m’yad doesnt work at all (see Chazon Ish when it comes to pulverizing dates) why woudn’t shinui be enough?
2 – what are the different methods to accomplish shinui?
The issues of shinui in tochen are a little complex, and I will write only briefly.
According to the Magen Avraham (320:7), it is permitted to grind for the purpose of eating right away, because this defines the act of grinding as an act of eating, and not grinding. However, if one uses a “grinding utensil”–and not a regular “eating utensil”–it is clear that one wishes to grind and not to eat, and this is therefore forbidden.
The Chazon Ish (Orach Chaim 57), is more stringent, and writes that even with regard to grinding for immediate use, a shinui is required from normal practice, so that if a knife is used to chop vegetables, the vegetalbes should not be cut into small pieces (as the Mishnah Berurah also mentions), or for mashing fruit the back of a knife should be used. This is not an “absolute shinui” (shinui gamur), but is sufficient together with the fact that it’s being done for the purpose of immediate eating. Rav Moshe (Iggros Moshe, Orach Chaim IV, no. 74, Tochen 2) writes that other poskim do not concur with the stringency of the Chazon Ish, but writes that it is nonetheless good to make a shinui.
The methods of shinui for fruit are using the back of a knife of spoon. Note that according to Rav Moshe, it is permitted (even for those who are stringent) to mash a banana in the regular manner (with the teeth of a fork), because there is no melachah of tochen involved in mashing bananas, which do not separate into pieces but only become mashed (unlike dates, which mash into pieces, even if the pieces then stick together).