More datapoints, this time with a ramp-flat-ramp \_/
First attempt: push from one flat square, travel across second flat square, then 3 down ramps then onto the lava-submerged 4th ramp. Didn't take lava, probably because some of the channel tiles were still 6/7.
Second attempt: the above setup retried, this time after assuring that all tiles are 7/7. Cart took lava but only made it to the half of the out-ramp, then back to the middle of the channel.
Third attempt: push from one flat square, 4 down ramps (no intermediate flat sq. this time.), 5th down ramp is lava. Cart crosses the bottom of the channel (not taking lava), climbs the out-ramp from which it takes lava (ramp fill goes from 7/7 to 5/7) but doesn't make it out of the channel, rolls back to the bottom as in the previous attempt.
More experiments needed but I sorta ran out of space for the experiments and also goblins have no respect for science. Will update after dealing with these problems