ICAO had acceleration before so it means that procedure was designed, probably very conservative, but still designed to cover all.
Operator is responsible for all contingency procedures but normal ones should be done according international regulations (PANS-OPS, TERPS....). If operator changes procedure or any part of it, it should be approved by relevant CAA and noted on the chart.
If you have some high altitude airport with terrain all around, how will you know at which point exactly your missed approach changes from intermediate to final phase for actual weight and conditions? Max speed for intermediate part is 160 for Cat C aircraft and Cat C can be A319 or A330. Between those 2 aircrafts, weight can be significantly different. Even if you disregard this, your acc alt of 1000 or 1500 will be further away for heavier aircraft and missed approach design gradient is always the same