a) I've run into invitations not being received a lot, both for me and from me, both via email and via Ancestry username.
b) I doubt anyone would have had occasion to try to test the link in a forwarded email but I see no reason why it wouldn't.
c) What does anyone's helpfulness or lack thereof have to do with this? Do you assume we would only invite people we think are going to be helpful to us? How 'bout if we're trying to be helpful to them?
d) Why should someone have to go open another email account because something that should work isn't working? It's not difficult to have an email copied to the sender's own email address, it's pretty much available all over the internet.
Not only that, I would like a copy for my own records of the addresses I've sent invitations to. Once a person accepts the invitation their email address is hidden from me as the tree owner and only their username shows.