If you made your brigade area too large you might well end up having command and control difficulties. There were reasons commanders formed brigasdes and divisions into two or three lines.
In regard to the situation it could well depend on the wording of the order. If a cgange of position can be justified under the wording of the hold order you are currently on then the change of position you mentioned might be permissible. If not you would have to change your orders. I agree there could be people who exploit the rule but, personally I think it would be reasonable to allow a zone of control up to say one move by the main unit type within that brigade. This would limit the degree to which exploitation would be possible while allowing some local flexibility. Hence Colbourne could flank the Imperial Guard as he did as long as he is still within a move of his brigade's assigned position (assuming the brigade was on a hold order. But when Wellington orders "the whole line to advance" the brigade would need to change orders to Move or Attack, whichever is appropriate under the circumstanes