300kpa? Those 4 person hulk ka?

First the tables for Load Index 82/88 and speed rating H/V :
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[attachmentid=10219915]

Car 1) 185/55/15 82V. Front 220 kpa Rear 220 kpa
Reading from Table 1,Load Index 82 at 220 kpa : load is 430 kg per tire.
Front axle:860 kg
Rear axle :860 kg
Total load on all 4 tyres :1720 kg ...... as designed by car OEM.

Now , from the perspective of GY manufacturer, this 82V tyre can carry up to 475 kg per tyre at a pressure of 250 kpa.
Maximum load on all 4 tyres would be 1900 kg.

Now, refer to Table 3:
This 82V tyre at a pressure of 280 kpa allows you to  drive at higher road speed up to 210 km/hr and, reduced load from 220 - 240 km/hr.

In conclusion , IMHO, GY manufacturer is allowing you higher pressure of 280 kpa carrying full load of 475 kg per tyre up to a max road speed of 210 km/hr.Reduced load thereafter from 220 to 240 km/hr.

Car 2) 175/70/13 82H. Front 210 kpa Rear 210 kpa Now using 175/65/14 82H
At 210 kpa, these 2 tyres (82H) allows for 415 kg per tyre.
Hence total load on 4 tyres as designed by car OEM is 415 x 4 equals 1660 kg.
Maximum tyre load permitted by GY at 250 kpa is 475 kg per tyre.

Again, GY allows tyre pressure of up to 280 kpa at a road speed of 210 km/hr, not beyond please note.

Car 3) 185/65/15. 88H Front 230 kpa for up to 3 persons, 230 kpa for 4 persons or more. Rear 210 for up to 3 persons, 300 kpa for 4 or more persons. 

Now this is interesting as there is discrepancy here.
With a recommended 300 kpa for 4 or more persons at rear axle , you are not supposed to use 88H tyres as it's limited to 280 kpa.
This is based on the assumption that the car OEM has correctly recommended 300 kpa on it OEM tyres probably rated at W or Y.
Can you clarify this ?

If your car OEM has correctly specify 300 kpa for rear tyre, then the OEM rear tyre (that comes with the new car) could not be 88H 185/65/15 ..... as H has a manufacturer limit of 280 kpa.
Can you kindly check your door side pillar sticker or owner manuals where W or Y tyre is specified and not H tyre ?

In this event, I hate to say 300 kpa recommendation is a blunder, big time from car OEM.
User manual may or may not have such details.
Generally pillar sticker should be reliable on recommended tyre pressure and tyre specifications.

Does the user manual or placard shows tyres being H, W or Y tyre  AND is the Load Index 88 or higher?
What about a picture of your pillar placard ?

Does the user manual or placard shows tyres being H, W or Y tyre  AND is the Load Index 88 or higher?
What about a picture of your pillar placard ?

So it says 88H tyres clearly on pillar placard.
Hence, you'd correctly used the 88H tyres here.
As to recommended tyre  air pressure:
280 kpa would be in compliance with tyre manufacturers/Standards Organisation, OR
300 kpa would comply with car OEM (???*which had no professional basis ,in a way) but non-compliance with tyre Standards Organisation.