Indeed, that's the exact way to think in 5e, simple and quick.
In any case, the DM cannot be cheating since he makes up the rules anway, but I completely agree that if they make sense, they will never cause a problem.
That, on the other hand, I disagree with. If the action is nonsensical, it should not be doing anything significant. But if it's clever or really well suited to the situation, it should provide way more than this. This is really why I did not like a large part of the 4e page 42 of the DMG, because - for the usual balance reasons of 4e - it normalised everything. After that, it's all a matter of taste, but this is where you get a perfect example of structure vs. openness. To each his own, some people will like the structure of 4e telling you what the damage is, as for me I prefer the openness of 5e which leaves the DM totally free to arbitrate.