I cannot say more about questions Q1 and Q2.
About Q3.
I have a PPPoE line, 1Gbps/300Mbps, MTU is 1492. My line is fine also without limiters, I had a solid A for bufferbloat, RTT is 6ms (first hop)
I tried limiters, using 1506 as quantum (1492 + 14 interface overhead), set limit at 7ms for download and 5ms for upload, bandwidth (950/285)
I tested with thoese limiters, set the floating rules as per netgate instructions, and now I have a solid A+ on bufferbloat test, with average speeds of 930/280.
I suggest to test against bufferbloat issues before using limiters, then repeat the test using limiters so you can see if they are working and improving latency management.