Hello. Simple answer - no. This is trespass and you can demand the supports are removed. If you don't, you risk the wall being deemed a party wall which limits what you can do.
Yes they are. I suggest you speak to your neighbour and ask for the supports to be removed and made good. If they refuse,
Put it in writing and say they have 7 days or you will seek an injunction.
Ok, well put it in writing as above. If he fails to comply I suggest you will have to see through on the threat and get an injunction. Also, speak to the builders and tell them to remove the supports and make good. They're likely to call the owner and tell him to sort it out with you.