First of all who is the housebuilder?
The whole purpose of this forum is to provide information resource to others and warn potential buyers of the pitfalls of buying from certain house builders.
My opinion is that all drainage access points should be on hard landscaping ie paths.
They should never be partly on a path and partly in soft landscaping like grass or borders as you say 'half and half'.
Obviously
something can be done. No one has died!
The sales advisor is no construction expert or guru! Just determined to be obstructive and unhelpful!
It is just a matter of time, cost and a desire for happy customers.
I would point out that unless it is moved so it is not 'half and half', you will definitely be giving the builder a bad survey score with a NO to question 1!
As for NHBC standards the only requirement is that they must not straddle a boundary or under a fence (no access, cover can't be lifted)
House builder's staff are not allowed to lie or mislead buyers, Not only is it against
Consumer Law, it also breaches requirements of the
Consumer Code for Home Builders which you should have been given a copy of!