Jez if i had a house to rent i'd want you as tennents. As for your questions:-
1) I think any landlord would love not having to bother with coming to cut the grass or tend to the garden, If he's having withdrawal symptons i have a big garden thats in bits and he can come and potter ( excuse the pun ) and do what ever he wants to it. Anything would be an inprovement on what it's like at the moment.
2) Again i dont think a landlord is going to mind if you decorate the room for your child, especially when your going to put it back to how it was. You might not even need to do this. When you move out another family might move in and love that it's already decorated.
3) Evertwhere i have rented have never had a problem with this. If a Landlord did i would find somewhere else. Although your only renting it is your home, and if you cant hang a few pictures and get a few nic knacks then it's not a home. Most landlords will request that you fill the holes and re-paint over them. I've moved into places where the nails were still in the walls, it was great didn't have to make my own.
4) yeah no problem with this. If there is too much in the house or you have your own things that you want to use instead, ask the landlord to remove it or put it in the attic untill you move out and then put it back where it was.
There are loads of properties available at the moment, landlords are desperate to rent their properties. I dont see any landlord having a problem with anything that you want to do, even before there was more supply than demand, so i really dont think your going to have any problems at all.
Good luck in your new home!