Dude, I understand your thought process and tend to agree. However, there are still considerations beyond the plan as you put it.
For example, lets say you have a plan that includes Mancini as your 1B for the next five years (or whatever number). But some team comes along and offers you the moon for Mancini. Well then you take the trade and revise your plan.
Having a defined plan is certainly the correct approach but those plans also have to be very fluid and flexible.
We know the approaches, needs, resources and ML and MiL rosters of all 30 teams in MLB. While you are theoretically correct that someone could talk you into anything (if they wanted him bad enough), I'd be pretty comfortable suggesting that's not a scenario I need to concern myself with.
I still think the best fit for Mancini in trade is DC. He's a quality player they could reasonably keep that would fit well within the other roster things they may want to do.
You'd need to project some team and package that would be considered the moon.