Well, I'll have a go at the portion of your question that isn't trim-related.
The positioning of your vents are intended to promote the circulation of air. In the south (of the US), air supply is usually up high and returns are down low to facilitate the settling of cold conditioned air. In the north, the heater vents are usually floor-mounted and placed in front of the windows. The returns are thus put high in the walls. Either way you don't want supply and return at the same altitude as this can promote short-circuiting the flow of the conditioned air (hot or cold). So, in your case, take a look at where the other vents are (you don't mention if this is supply or return air or which is more significant in your climate) and decide how far you can move the vent without getting "too" close to its opposition. Generally, a few feet in any direction is not going to make a significant change in your HVAC system.