OK, I think I figured both out.
The Agentuino lib does NOT like receiving another GET request before it has responded to the previous one. The workaround was to increase the time-out value to 1500ms in Cacti. This causes Cacti to wait 1.5 sec for a response (instead of the default 500ms) before re-transmitting the request. Since the Arduino does reply within 1.5 sec, Cacti does not need to re-transmit the request. With my current code the Arduino takes just under 1 sec to reply.
It would be nice if the lib could handle this a bit better. But I guess with 2kB of RAM, you can't buffer more than one packet?
The problem with the values being in the millions was solved by specifying "GAUGE" as the data type in Cacti. Before I used ABSOLUTE, which seems to interpret the data differently.
So that was not an issue with the Agentuino lib.