Thank you, Grant. The pointer to Rover parameter documentation evidently filled a blind spot for me. I mistakenly relied on Mission Planner (v. 1.3.44) built-in parameter documentation, which misled me to choose RNGFND_TYPE=4 for PX4-I2C and RNGFND_TYPE=5 for PX4-PWM.
In any case, using RNGFND_TYPE=4 resulted in the wildly erratic motor-drive signals from ArduPilot that you can see in the log files or RCout graphs that I posted, which are certainly not present when the LidarLite was not operational (i.e. RNGFND_TYPE=0), even with NAVL1_PERIOD=8.
As a matter of fact I did run a series of trials my last day in Florida, with NAVL1_PERIOD set to 5, 20, 30, and 40, to confirm a previous probationary determination that NAVL1_PERIOD=30 worked pretty well for us; I used NAVL1_PERIOD=8 for the LidarLite trials only because it was the "default" value (albeit for land Rovers, not water-borne Rovers) and I'd observed quite acceptable, stable behavior with that setting without the LidarLite interface.
On arming checks: forgive me for saying, but it is a code feature that I felt I could do without. Beginning with Rover 3.0 (after 2.5), I found that I had to set ARMING_REQUIRE=0 to run at all, so I became disenchanted with what seemed like code overburden. Also I worked quite a bit trying to effect remote arming (for the purposes of starting a new log file and setting "HOME" since it always reset to the location where arming occurred), and was unable to arm the Pixhawk any other way than by the hardware switch.
Unfortunately I'm not able to do more water trials with RNGFND_TYPE=15 where I am now.
Thanks for all your help,