So a question...

It looks like there's no risk to buying enhancements. I can "unmount" them at drydock for full resale value - is this correct or am I missing something?

As an example, let's say I "buy" a shield enhancement worth 30K. I can then mount another 30K shield enhancement at drydock for no cost. It simply swaps one for the other (I'm not charged again for a new 30K enhancement, the game simply swaps out my old for the new because they have the same cost and I can only use one at a time).

Likewise, if I keep my 30K enhancement and wander around for a bit, it seems I can go back to drydock later and "Unmount" it for full cash value. What am I missing? Or should I always just slap on as many enhancements as I desire, because there's no risk / loss to unmounting them later for full value?

I'm used to RPGs where, if I buy an axe, that's it I lost the "gold" used to pay for it. Here it seems like I can apply full value for that "axe" to a new shiny longsword. Is this a glitch, am I missing something or is this a conscious design choice to encourage players to mess with various builds?

You are correct.
We have many people trying to achieve the perfect build and become almost invincible due to cumulative bonuses of officers, items and enhancements. :)