Car losing power and totaly shuting down

While driving mine Seat Ibiza 04 1.4 petrol (60 000 km under it) few days ago, first the A/C shutoff, then the ABS light came on, lost power steering, then the car just shut down totally and would not start again. There be enough of power that the windows would work, radio would turn on, but the engine would not crank. When it happend i had no power to turn since the power steering was dead and i couldnt brake since there was no engine power to make them work. I think i had huge luck on my side that i didnt crash. After that i had to disconnect the power from the battery so the board pc could reset so i would be able to start the car again. Took the car to the mechanic right away and they did some stuff with it, they didnt replaced anything just cleared up some errors that showed up from the board pc, they said that the car was recharging perfectly and that the alternator was ok, and also mentioned that the battery was giving out voltage below the required value, plus they also mentioned that ill need to take the car back after a couple of days so they could Flash the board pc, didnt said why or how would that help. The battery is almost a year old so i took it to the dude i bought it from to be tested, it showed that it was in good working order. The reason i got the new battery was that the old one started dieing if i dont run the car for 2-3 days. At the beginning the new battery was doing its job but after a while it started dieing the same way as the old one if the car was not run in couple of days. Im about to take the car to another mechanic where my friend works at, so they could work out the problem cause i dont really trust the previous mechanic even dough they are the official service shop for the Seat brand in my country. I turned here since i wanted a feedback from some of u here that have some exp about similar or exact cases like mine, on what could be the origin of this problem and its possible solution. Im really worried about the power steering loss and the inability to brake since it could prove fatal at high speeds. Every opinion would be deeply appreciated.