Adding the diffie-hellman line to the config worked, but then the router complained that there wasn't a matching cipher.
I suspected that if I fixed that, the router'd complain about something else, so I gave up and fixed things by copying the working /etc/ssh config dir from the laptop to the other hosts. I compared the working config with the bent one and didn't see much of interest. I really have no idea what I did to break things. But it's going. Thanks all. -- Glenn English