Just buy the extra 2 NC contact blocks and put then in the enclosure, then you can break both line and neutral with it safely. 3 blocks because I have found that a lot of Telemechanique and KM switch blocks are very unhappy with having 2 contact blocks attached, and do not like having the centre empty either. So 3 blocks with one being unused are fine, and as well you can also wire the centre with a NO block and have a red LED lamp ( use a 3 station button housing, very common) to indicate button pressed, and a green LED to indicate power on.
If you want extra safety use a bigger housing ( deeper, but you lose the extra holding that the switches get from the base there) and use 2 switches in series on each phase. That way a welded contact will still be disconnected. Those contact blocks, while rated to carry 16A, are not really rated for long life making or breaking 16A though. They will fail, and there are very few 25A rated switch blocks available any more, you have to go with a low power contactor ( with the 9VA power consumption) to get that.