Technically, it is the same as both Toyota and Honda uses mainly ethylene glycol based coolant.

The only difference is the colour of their dye. Red vs Blue.

I would say there is no major concern as both are interchangeable but to avoid hassle, I will always stick back to Honda coolant for ease of monitoring the colour of the coolant.

Once you've mixed red with blue, it will be difficult to monitor the colour now unless you do a 100% coolant flush.

Total how much the SC charge to flush out the entire coolant for you?

I would suggest you to top up using distilled or battery water.

Topping up with coolant will increase the glycol content and you will not have a 50/50 mixture.

Yes.. initially I am having the same thought as you. However, later I ponder about it.

The reason coolant loss occur is mostly due to heat evaporation. Means that water is being evaporated but the mineral or chemical content will still remain in the car's cooling system. Just like sea water being evaporated and the remains will be salt.

So to maintain the 50/50 ratio, we should add in distilled or battery water. However, I will still drain the coolant after every 3 years with 3 fresh bottle of Honda Type 2 coolant.

That is my opinion.