Instant heater will have poor flow rate especially in the winter.
As Shaun, far better to have an unvented cylinder.
If it’s going to be a self contained granny studio I would definitely heat it from within that annex - even if this is electric only.
A dual immersion cylinder is good for this...
see if you can drop the pipe down into the porch and bring it across the inside above the archway. Then drop it vertically down inside the corner and bring it back outside to the stack at low level.
You talk about floor supports, what are they and what are their dimensions?
400 centres does not give 400mm between supports unless they have zero thickness!
I find it’s often helpful to tap the handle with a non- marking rubber mallet as you pull it upwards.
You may also find it to be easier to remove the handle if you remove the tap from the basin.
The absolute best thing to do in these circumstances is ask your local council/building control - you can do this anonymously so as not to compromise your position.
I would follow the manufacturer’s instructions.
It depends on how it goes together.
I have fitted a similar unit by assembling it fully in the middle of the room as it was very useful to have access all around.
Tip : first try fitting the base into position on its own and spend some time...
As it’s new build I’m surprised that the rain water seems to be combined with the waste?
Round here they would definitely be looking at disposing of rain water to a soak away, to relieve pressure on our overworked sewerage system.
That’s why I say you need to establish exactly what goes where...
You need to find out for certain what this pipe is used for- not just guessing.
Then, if it were me I would re-route it so that it would be outside the area of my proposed extension.
( I wouldn’t bother if it was used as a conduit for cables for instance)