You can take the cover off and screw the frame to the riser. But if there's ever a day when you need into that chamber (as sewage is backing up), you will want the easiest access possible, not fighting with a stuck lid.
You could weigh it down with an old brick so you don't knock it off
You don't need to concrete decking posts in, if you're burying you can backfill with rammed earth/stone/gravel. Or you can have the posts non buried, sat on a bit of a paving slab or whatever
Cutting a hedge for someone in a more urban area, I found a police "no waiting" cone inside, it was totally hidden, and a real fight to get out. Think it had been launched in from the top
Next time you throw out a broken electrical appliance, cut the lead off and keep it!
The camera/light will no doubt have a tiny draw, so I'm sure a 0.75mm2 lead will be fine. It should say the power rating on the item/box?
The old saying I've heard was along the lines of the thickness of your board is the size of the gap.
So if the boards are 20mm thick then the gap is 20mm.
If you build it with rough sawn timber, and the boards butted up against each other, they will dry out leaving a gap which will allow some...
Pinch bar.
https://www.dwtoolshop.com/products/bulldog-bcb72cp-72-x-1-18-chisel-and-point-crowbar
If you want to splash the cash there's various "jack hammers", but it can be a bit of a slippery slope once you start looking at longer chisels, anti vibration etc
Best way as in easiest, or cheapest?
If you can sort it out you might be able to get rid of a lot of it for free on gumtree etc
Soil for infill
Stone/bricks/concrete for soakaways/rubble/infill
Wood potentially for burning
Anything else fill your wheely bin every week
What type of fencing are you wanting to put up? You can get various types of materials - steel (plain or galvanised), concrete, different species of timber; spruce, larch, oak, chestnut, creosote treated, etc