I would have to visit showrooms to see if newer models (if they exist at all), have any difference.
Yes, that would be best bet. However you need to asses whether you are so much concerned about the multitude of temperaure options available.
But these temp settings are mentioned for Convection mode only, but not grill. I don't understand why.
You need to understand how temperature control works in convection mode.
There is a heating element behind the cavity wall next to the circulation fan. This heating element switches on, the fan is switched on and the air temperature inside the cavity is measured. Since the air in is constant circulation, the temperature inside the cavity would be nearly uniform. Convection = heat transfer by moving hot fluid (air in this case).
Boiling & steaming is also convection mode where heat transfer is by moving hot water / steam.
When the temperature element detects the air temperature exceeding the set temperature it cuts off the heating element, but the fan still continues. Slowly the temperature drops (due to heat loss to ambience + due to air getting cooled by the food being cooked). As soon as the temperature falls below, the heating elements cuts in. This on-off switching happens till the programmed time period. This operation is very much similar to air conditioners at our homes.
Now the grill is like wood/charcoal. It is supposed to be ultra hot (the actual gas/ wood/charcoal burning surface could be as high as 1000 deg C). The food is kept close by and gets heated by radiation. This can happen in the absence of air also. The air may be in movement (breeze) or closed natural circulation (wood fired oven) or practically stationary (grill where hot air rises vertically) - but that is not the main mode of heating. You are supposed to visually judge and stop/start grilling
Of course, the OTGs do have temperature control so it could technically be done in grill+microwave, but OTGs require temperature control for "baking" purpose not for grilling or toasting purpose
So for heating = microwave oven, for grilling+baking = otg? Or, is otg good only for grilling?
For heating microwave is king.
For baking convection is better than OTG (and air fryer is the best) because of the circulating fan that improves the heat transfer.
For grilling having largest wattage element is the best (you need to see whether your preferred OTG vs grill/microwave vs 3 in one). The placement of the heating element also matters: Most micwave/combinations have only the top, most OTGs have at the top sides and bottom sides, few Panasonics have top and side (labeled as magic grill).