"It's not a bad start to the model, i'd like to see it finished.
Back to your question, modelling like this CAN work, it's just that while building the model you would have to make sure that every single part is on the same offset point, then when you mirror the parts they will all mirror around the same point, making the model look correct.
It won't work with your model anymore, this one will have to be finished manually, but keep in mind what i said for the next one and it should work your way!"
it is correct. mirroring is not changing the rotation. the reason is simple: the rotation point remains the same on the mirrored part and so the rotation would screw up the model.
that is the reason why i build most of my model without using rotations.