Gravel edging is usually put against walls to reduce splash up the wall from rainfall [vs a flat surface]. I am not sure if it achieves this by stopping puddles forming, or because of the rounded surface.
You compact the base to make it flatter and more stable. It will give you a more even finish.
There is no norm for the size of gravel, you choose whatever appeals to you visually. Why do you think it needs to be in a grid or glued down? Have you considered corten steel edging between the grass and the gravel?
I did this - I used 2x what you have, next to each other. One is fixed to the top door and the other is fix to the bottom door. Whichever door handle you pull on, pulls the other door with it.
When done, put something synthetic on the bottom of the legs to keep the timber off the ground. This will stop the end grain soaking up moisture from the ground and prolong the life of the legs. I used these on a particularly hefty bench I made:
I am now not sure if the conifer is a leylandii, it's weeping and the leaves are a bit frilly. But probably prunes the same.
As for the cherry...that might not be a cherry either...my plum has bark a bit like that.
Pretty sure that's a swing though :D
A quick google tells me you can prune...
You need to do some pruning. Ideal time is winter when everything is dormant and it's easier to see the structure of whatever has lost its leaves.
The cherry doesn't look too clever.
The shrubby thing with a lot of small leaves looks like cotoneaster, they [can] have interesting foliage in...
Where do you get a straight bit of timber? :)
I am not saying the linear clamp thing is useless, just a little less useful than expected. On mine at least the extruded aluminium section appears to be perfectly straight.
I also have a very long sash clamp, which I can use as a straight edge with...