2) what has to be done to the "data" pair of a peku port to make it play nice? searching says "short it" or "put a resistor across it" but... is it really that simple? does that trick the processor into thinking there's an ekt phone there, so when I "call" it, it thinks there's a phone there, and lets me talk, like if it was a station to station intercom call?

Yes, it really is that simple. Phonemeister is correct. I think tech support used to say to use a 10 or 100 ohm resistor across the data pair but just a direct short works fine.
Adding PDKU's and digital phones requires the PCTU to be a rel 3 or 4.
Most programming can be accomplished with a 10 button set, but a 20 button display is ideal especially for reviewing existing programming.