You mean put tint film on top of the existing one?

Can but both film will void warranty. The previous one will say, putting another tint on top void warranty. The second one will say, the tint film is suppose to be stick on glass

I suggest you replace the old one with new one.

JPJ current rule for VLT is:
70% front
50% driver and passenger side
30% backseat windows and rear

I'm using 70% front and 50% for the rest. This is still in line with rule. Some people go for rear seat windows 30% but it look weird to mee. Not even between front and rear windows. For just read windscreen at 30% is still ok, but for me I stick with 50% because I like to see what behind me more clearly. I always monitor surrounding from time to time when driving to be alert. If too dark it is bad especially when it rain and at night.

Why want to use 75% or even more VLT 85% if JPJ allows down to 70%. It will be hotter. The TSER for 70% VLT for good one is probably range in 50~60%

My bad.

Look at it this way. It is not about whether it can cool faster or not but different point of view. The sun is continuously shining on the car along the way. With more heat rejection, the aircon will kick in less often compare to the one with less heat rejection, thus reduce fuel consumption

Anyway, the IRR is not everything on heat rejection. The TSER (Total Solar Energy Rejection) is the specification you want to look for if provided. The IRR can be more than 70% but TSER is only 45% for example. Go for highest TSER within budget.

Worth as long as you feel good about it.

Thanks for asking because it makes me wanted to know also. Here is what I found out. I take an example from 3M Crystaline series. Here is the data:

CR70
TSER=50%
IRR=97%
UVR=99%
Glare Reduction=22%

CR60
TSER=53%
IRR=97%
UVR=99%
Glare Reduction=32%

CR50
TSER=56%
IRR=97%
UVR=99%
Glare Reduction=44%

CR40
TSER=60%
IRR=97%
UVR=99%
Glare Reduction=55%

CR20
TSER=62%
IRR=97%
UVR=99%
Glare Reduction=77%

Maybe it will not answer to your satisfaction but ut seems to me even the small percentage is probably significant. See how the IRR and UVR number did not change at all. Looks like it is not easy to gain the TSER but do you think the CR70 vs CR50 will not have significant heat reduction? The TSER difference is only 6%. With CR40 it is only 10%. I guess it might be significant and something you can sense in real world. Just my opinion.

It should be min instead of max

No idea. I never use 3M