Question: If we run the microwave (which has a convection feature) on "grill and convection" mode only, does it become same as rectangular "Electric ovens"? Please explain why and how too.
Soegaard already explained the difference between heating by microwaves and by standard oven. The point is, they are completely different methods, and the same oven needs two types of heaters within the same body to have both settings.
Normally, I would assume (the same as soegaard does) that if you put the oven to the "convection" setting, the microwave heater is turned off. But the truth is that it is up to the manufacturer to decide if this really happens, or if both heaters are turned on in this setting. It surely sounds illogical, but we have evidence that it does happen - and the evidence was for an LG oven. See this question: Why do my pizzas get such hard crusts?. So we can't promise you that it is OK to bake them in your oven, and I suspect that a manufacturer implements a feature in the same way across ovens. So assuming that the oven in the other question works as it should (it could be a defect unit which doesn't turn off the microwave heater when it is supposed to), you have low chances of getting good cakes.
Does it make sense to implement the feature that way? If you aren't baking, probably yes. You see, in a microwave, you don't get a crust. Pure convection cooks slower than microwaves. If you are cooking a veggie casserole in the oven, you'll probably be happy to have convection and microwaves at the same time. This still doesn't explain why the manufacturer labelled the feature in this misleading way, or why it doesn't give you two separate dials, one for the microwave heater and one for the convection heater. Maybe the target customer group are people with minimal cooking skills who only use their oven for reheating prefrozen food.
So we can't tell you if your oven will bake a good cake or not - it depends on how it is built. The best way to know is to just try it. Make a normal bread dough - don't waste time on preferments and the like, go for minimal effort and expenses - and bake a loaf in the oven at the convection setting. Wait 2-3 hours o give it a chance to harden. If it tastes like a nice bread afterwards, then the microwave heater was turned off and the cake will bake well too. If it is unusually hard - microwaved dough is practically impossible to bite off - then the microwave heater stays on at the convection setting and you can't bake a cake either.
You may want to call customer service before you try, and ask if the "convection" setting turns off the microwave heater. If the person on the other end of the line knows for sure how the oven works, they can save you from a useless experiment. But there is always the chance that the call is a waste of time.