We have agreed to not let him on the sofa. I think he wants to get on as thats where the kids go to be able to see him but out of his reach. He isn't actually able to jump up yet.Sounds like you are getting on wonderfully well! You will find that a routine develops then like gets easier because you start to learn when you can relax and when you need to be aware.
You must decide the rules, whether you allow him on the sofa or not. Personally I do, but I'm on my own and if Chloe didn't use it then nobody would! But if you decide you dont want him on it then the easiest way is to make it difficult for him, put something on it so there is nowhere for him to get comfortable. OK, it makes the room look a bit of a jumble, but it's only until he gets the message so it's worth a jumble now to have what you want later. As to going under, block his access! I had to with Chloe, but they grow so fast that within a couple of weeks she was too big anyway so the problem was over. As to chewing, there are "Anti chew sprays" Bitter Apple and the like, but check on a piece out of sight first that there is no staining.
Sadly this is pretty much a Labrador thing. Maybe a puppy pen to limit his access to the children. (Particularly when he gets overtired and fractious?)
Eating grass in it's self is harmless. Most dogs enjoy "Grazing" but a blade can get stuck in the throat and make them sick. So yes, discourage it, but dont worry over much. Old fallen fruit can be a problem, particularly in summer when the wasps are around. I had a lovely Victoria plum in the garden which my dogs loved!
I need to find something to block under the sofa, but I don't have anything he doesn't pull out immediately.
He's also taken to jumping off the wooden decking onto the grass, which is 2 steps and it's not far but does make my wince when he lands. Obviously not encouraging that.
We have a big plum tree and an apple tree, we do get wasps and bees in Summer, so we'll have to watch that.