You may already have discovered this, but don't press too hard on the panes. They are generally thinner than house window glass and may also be brittle from age.
There's no sealant that I'm aware of that can be used on a pond in a way similar to sealing a bath. The first problem is that most sealants require a clean dry surface to bond to, and you won't have that with a leaking pond. Secondly, there is likely to be so much crud under the stones that there won't be a solid surface to bond to anyway.
The only suggestion I can come up with is to use gravel trays partly filled with water and stand the plants in there. Shade the greenhouse as much as possible and leave door and vents open. Cross your fingers beyond that.
We have no idea why the OP wants/needs to pave over the garden, and it's their garden anyway.
I've tried putting signs up asking slugs and snails to leave my hostas alone but they ignored it. I have now started to write the warning on little blue pellets and it seems to do the job.
Just keep pulling out what you see. There's unfortunately little to be gained by trying to dig it out, as all that does is break it into smaller pieces, each of which can regrow independently.
Laying paving directly onto grass, without any firm base, isn't going to work in the medium to long term. After heavy rain the ground will soften and any uneven areas will become more apparent and that will get worse with every season.
Malvern Spring Show coming up this weekend. Rain therefore just about certain.
To the best of my knowledge there is no specific 'right to light' in the garden.
I have found this which might be of use once the hedge grows.
If the conifers grow beyond the boundary of your garden you will be within your rights to cut them back. If you are saying your neighbours have actually planted them on the boundary line you would be within your rights to tell them to move them and replant them in their own garden.