Yes, You inlaw could
(#2 Can he connect to the DSL modem without a router?)
bypass the Airport.
He will need the Telco DSL installation Software, often available at their website if you cannot find CD, the username, and the password. You could plug your Airport when he is done.
The VPN IPSEC (security measure most of the companies employ) should work either via wireless or wired connection as a client, however, this way he/you could eliminate the possiblity that your Airport is causing the problem.
A switch is a very cheap and good thing to have if you have a multiple number of cables to plug in, I hardly think it could mess up your Airport configuration. Good luck!
