WH will require minimum 4 sq mm cable with RCCB 10ma protection  -  this is Suruhanjaya Tenaga rule!

Your water heater if on at full blast (max temperature)  will melt the cable and the 13A socket plug.

If you insist to use back the same wire you must minimally install the 10ma RCCB and make sure your RCCB is 16A at maximum otherwise your cable will melt and catch fire -  the cons is you should not on your WH at max temperature because your breaker will trip to protect your wire from overheating.

Still 13A socket point should not be used here,  go buy the one specialized for WH.  This is a matter of life and death though have fun arguing with some half-past-six electricians that will suggest with you to go with 100mA RCCB or even 300mA to prevent tripping -  30mA may be the best advice they could give HOWEVER 10mA is A MUST by ST rule!

Do not forget -  Insurance might choose not to compensate if they find that the accidents/fire was caused by wiring setup that did not follow the right specification!

2mm cable? Not 2.5mmsq for the in-wall concealed cable?

Its OK but you might consider using a CEE plug instead of DP switch (so u can disconnect the oven)

Using non-sirim product is against the law - you can try raise a complaint to ST if you want but make sure you do your homework and have the necessary documents/information - and also if you think it is worth to go through the "process". It is completely up to you... that's why there is always a phrase of "prevention is better than cure".

Using a non Sirim cable you would expect the cable quality is at least 30% bad (just cincai plucking from the air, but who knows may even be worst). There are videos in YouTube which show you in true practical test - just search it.
Sad to say this is the Malaysia "norm".

Judging by my own experience going to 10 electrical shops giving me bullshit theory (RCBO no such thing, 30mA water heater, 100ma for socket etc) - I would say that this is so normal when wireman decide to also follow the "norm", partly also driven by customers who only wanted cheap.