Much of your post is just wrong. The trains like any new trains, would have been procured against a published requirement including performance, safety and other parameters. Apart from the obvious performance requirements, (speed, capacity, passenger environment etc.), there would be a whole plethora of legal/safety issues defined such as operating with defined signal systems, power supplies, failure detection and recovery, etc.. Many of these requirements conflict with each other to the extent that one can be achieved at the expense of the other, e.g., power efficiency in contemporary EMU designs tends to require an ac conversion to a DC bus from which a three-phase inverter feeds power efficient ac induction motors. This involves a lot of high-frequency high-power switching electronics that unless carefully designed, would seriously interfere with other systems both on the train and trackside. Sometimes, the careful design techniques required not only increase weight and physical servicing accessibility but also energy efficiency of the traction system. However, interoperability of equipment (including equipment not under the control of the railway is mandated under Electro-Magnetic Compatibility legislation which uniformly applies across the EU specifically and in some form or another all of the developed world.
As has been mentioned in other posts in this thread, this power supply problem was different to any of the previous major failures in that the 50Hz frequency varied outside established norms and varied at greater rates. If as I believe, the relaxation in the supply specification was not included in the specification for the trains, it wouldn't have been tested for during design qualification. If so, then the reviews that have been kicked-off will identify that, and presumably a) recommend a suitable design update to deal with it and b) revise the process of creating technical requirements to prevent a repeat of something similar.
As far as ac traction being synchronised with the grid, no, that is not the case. As I said above contemporary EMU designs tend to have a DC bus, conveniently at or around 750VDC. The 3 phase power to the traction motors is variable with the speed at which they rotate. This has been true since the need for all new EMU designs to be potentially suitable for either OLE and/or 3rd rail networks. Thus (I believe), all Electrostars, all Desiros, all Aventras, the class 395s and probably the new stock currently on order, (e.g. CAF 331s, Stadler etc.), have a DC to 3 phase ac traction inverter fed from a 750VDC bus. During manufacture, they can be fitted with pantographs, transformers and rectifiers for OLE operation and/or pick-up shoes for 3rd rail operation. Some types have both, e.g. class 377/2, /5, /7; class 387, class 350/1, class 700 and class 717.
It's also worth mentioning that 1980's designed class 319s also have a 750VDC bus, because of the Thameslink route's dual power systems although they have DC motors.