I will try to answer your questions as i am also using these meters.
- Pulse counts correspond to the #define PULSE_FACTOR 100
- The value in the upper right corner is the usage of that day (in your case 0.230m3 or 230 liters)
- yes, you could edit the meter in the utility tab of Domoticz to add an offset but i don't use it so not sure.
- Correct! the SEND_FREQUENCY is the interval in milliseconds (so 300000 is 5 minutes). You could lower it. I am currently working on a more instant update for the flow rate. I will test that after i calibrated the sensors and implemented the new library.
- I am not sure, but I think this is something of Domoticz and not MySensors. Maybe there is no gas flow widget and the water widget is reused.
The edit and offset option also works for the kWh meter and Water meter.
Hope it helps.