Line of sight means, as you suggest, that there may be issues with trees in the way.
You’ll just have to wait and see what the installer says on the day.
If the base of the DP pole is buried in a hedge, then a hoist my be required to access the pole top, as no safety check can be made at the bottom.
Conversely, if they do need a hoist, then that might help if there’s branches in the way of the span.
The grey demarcation box/customer splice point doesn’t dictate where the ONT goes. The fibre from the ONT will be run back to this box.
As long as your proposed ONT location is reasonable, with a clear run back out, you should be OK.