Adhesive primer? and toilet box backerboard joinline

[Sean-tries-to-tile]

Hi guys, I would be very grateful for some advice about wether it is necessary to use acrylic primer on the walls in my cloakroom toilet wall. I ask because to me it looks grey like sand/cement mix? Its a 1955 council house. The walls aren't smooth like normal plaster elsewhere in the house.
Also I have 12mm backerboard to fit to the back-to-wall toilet box with drywall screws that I plan to tile directly onto... the sheets aren't large enough to cover in one sheet so there will be a vertical or horizontal join.
Where is it best to have this join line? (Vertically in line with the flush plate and toilet pipe holes, or would that be a weakness? I don't want the thing to flex and break the grout lines!).

Tiles will be 7.5 x 30mm
what is best adhesive depth for those tiles?
3 Questions there! Sorry I am a complete novice but I plan to take the time for a top finish.
Many thanks in advance and I can send more photos if required. Any pointers at all will be very appreciated. Cheers

 

[Sean-tries-to-tile]

Keen to get started here (day off work!), maybe I should just prime, level out the 4mm uneven areas with adhesive, wait to dry a bit and start tiling?
I think I'm just waiting for someone to give me the go ahead so I can crack on (or point out any glaring errors so I can rectify!) Cheers

 

[Sean-tries-to-tile]

Just in case anyone was tempted to reply...😅 I have just been told it could be seraphite on top of sand/cement

 

[Rustytabitha]

I hope you sorted it I didn’t get reply i needed either

 

[Sean-tries-to-tile]

I hope you sorted it I didn’t get reply i needed either

Still waiting on best adhesive depth for 7.5x30 8mm ceramic tiles? But I have learnt (hopefully true!) that acrylic primer can be used for plaster and cement substrates.