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as in under the lip at the top of the bath? if so, it appears to have come prepared as there’s batons of sorts around the perimeter already :)
hvsds-
Fix the 2x2 to the wall and rest the bath on it. Not sure if you have batons that replace the 2x2 provided by the manufacturer, I've never had any come with a bath.
It's just that people started to install them with the legs only and they need to be stuck solid to the wall and have something to rest on.
It does depend on the household but you could be supporting quite a bit of weight and it's not unheard of for 2 people to be quite active in a bath so it needs good support.
CT1 is an adhesive sealant and you need that there to protect the wall when the decorative sealant between tiles and bathtop stops working. That way in the future you just replace the decorative sealant.
Airhead
You should put 2x2 under the edge of the bath fixed to the wall and fill the gap with ct1 or similar. You should do this with the bath 'full' of water preferably.
I usually make a braced 2x2 frame for the open side, like you would do for a stud wall.