Air Asia — Check in service

I have been a traveler with Air Asia for some time now (years) on flights from Bali to Perth return.
My family and I in total have done around 30 flights with Air Asia in 2016.
On my last flight I was far from satisfied and I want to bring to your attention the following points. I would appreciate an answer or some reasonable explanation from Air Asia management on all the points below.
1. At check in on my flight today (Jan 3 2017) Qz 545 from Perth to Bali with my family we were, between the 4 o[censored]s, 11kg over the 80kg allowance in the weight of our check-in baggage.
The check-in staff in order not to charge us for excess offered us the solution of moving some of our check-in weight to our hand carry.
Our hand carry was less than 10kg total for the four o[censored]s. The allowance of 7kg per passenger then meant, by my calculation, that we were actually under the hand carry allowance by some 18kg. I called for a manager and pointed out that with the check in and the hand carry we had that the total weight of all our baggage i.e. what the plane was to carry was in total for four o[censored]s 102kg and was actually below our allowed total of 108kg.(80kg check in and 28kg hand carry)
The manager gave me some convoluted reason, which I could not fully understand, that it was not the same thing and we would still have to pay an excess at $20 per kg.
Surely no matter in what form our luggage is in (hand carry or check in) the weight we bring aboard the aircraft that must be flown is still the same. Can you please explain why this is not true?

2. Now the other weight part that I still find hard to understand:
I asked the same manager why each member of my family who all work hard to maintain a healthy body weight and are well under the average body weight of a passenger are then penalized for having less than 3kg of extra weight in our luggage.
The answer I was given was that the passenger weight is averaged out across all the passengers on the flight. If this is true and as I understand the cost of fuel for an aircraft flight has a direct relationship to the weight it has to carry it seems then that passengers like us who are an average weight are subsidizing other passengers who are overweight.
Why should overweight people be allowed to fly without paying their fair share of the fuel cost?
I am not the first to complain about this inequity and I am sure I will not be the last.
The solution I believe is very simple, all the airline has to do is to allocate each passenger a total all-in weight. This total weight would include average body weight for height based on international standards and the amount for total luggage, both hand carry and check in.
If at check in when weighed a passenger’s total allowance was over this weight then they would pay overweight per kilogram based on that additional weight.
For passengers under their total weight allowance they should get a refund as the “spare weight” allows the airline to charge those passengers who are overweight without compromising the maximum safe load the aircraft can carry.
I think the logic is reasonable and I would like to hear why it could not or more importantly should not be implemented.
3. On another point can you please explain why when we were called to board the plane we spent 15 mins in the crowded air bridge before being let onto the plane. Surely ground staff should only call for passengers to board when they can enter and exit the air bridge directly onto the aircraft.
4. I also asked for a drink of water on the flight and was told I had to buy a bottle of water which was more that I wanted or needed. Surely even with a cheap airline the supply of drinking water to the passengers is a fundamental service which should not be charged for?

We are booked on our next flight in February, I hope you can answer these questions/comments before we board that flight
Yours sincerely
Richard Manser