Can the “Welcome – Stay in touch with colleagues and friends” message on LinkedIn home page be removed

You could try unsetting your message receiving settings and then setting them back. I changed my email not too long ago also, and have email still going to my old address (which is not a problem for me). I did notice upon joining a new group that the group digests go to my new email address.

If he was your 3rd degree connection, this does not mean you now have 'friends in common'. That would have made him a 2nd degree (you->common friend -> him). What you are looking for is a person whom he and another one of your friends have now in common.

Or to put it in different words - a 2nd degree connection which at least two of your friends share.

I don't know of a way to get this through to the interface. The nearest is to go to Advanced Search, chose 2nd connection and sort by connections. This should show at the bottom how many connections you have to that person. You are looking for two or more. But that's still thousands of people in the 2nd degree network. And you have to expand the connection list every time to check whether your new friend is one of those two+ connections.

Otherwise, you have to use LinkedIn API but that does not seem to be a very friendly interface.