I agree with you with regards to edge cases. I do not believe the nature of flaming sphere to be one of them. It's a low level spell (so likely to see frequent use in-play) with obvious tactical applications that vary wildly in their nature and effectiveness in function with the spell's definite nature.
Well, it's just one spell out of, like, 300? Seems narrow enough to be an "edge case" to me.
Granted, they could have fixed this particular goof with one sentence. "The sphere's space counts as difficult terrain, and any creature entering the space or touching the sphere must make a saving throw against its damage, as though rammed."
I have an intense dislike of trying to apply logic to magic - since it immediately becomes "logic" (which I abhor).
I totally agree, but
you still have to make a decision somehow and with zero process or guidelines for yourself you will always be lost and relying on the designers to solve as many edge cases as possible.
For example, how do you decide how NPCs act and what their attitude is? That's not logical and scientific, but with a halfway-decent grasp of human nature, you can decide what the NPCs do. When I am not sure what an NPC would do (people are complex and mysterious) then I just decide the Charisma check DC for the players to convince the NPC to do what they want instead of the alternative.
So, the nature of magic. If magic is not science, what is it? I've thought about this quite a bit. Here are some options I've seen used in fantasy books, and how I would use them as guidelines:
1. Magic is emotion. What desire fuels the flaming sphere? Would such a desire obstruct passage or not? (A flaming sphere is a slow burn. I'd say anyone who enters the sphere's space has their speed reduced to 0 until the end of the turn.)
2. Magic is metafiction. Would it make for a better story to allow the sphere to block passage? (Yes, but it's even better if the sphere is easy to push -- but you get burned automatically.)
3. Magic is power, and its cost. What does the PC gain if the sphere is solid? What do they risk by having such a flaming sphere? (Maybe the sphere has a mind of its own, or becomes insubstantial when it's least convenient.)
4. Magic is patterns or attributes, transferred to a different subject. What does a flaming sphere resemble? What concepts is it made of? This is the most scientific option, but it's really more philosophical, and is the closest to real-world views on magic. (I'd say the sphere is like a rolling boulder, VERY solid.)
5. Magic is inherently mysterious. If the characters in the story accept this, it's a fairy tale; if they fear it, it's supernatural horror. What could make the sphere more creepy and ominous? (Maybe each creature the sphere kills makes it more solid and harder to push past. Maybe you just randomly roll to see how the sphere works. Each time you cast it. Maybe if yo do a certain thing the sphere becomes solid, and if you forget, then when the spell ends it comes after you.)
My point is, having a coherent set of guidelines goes a lot farther than specific rules. In fact I tend to view rules largely in terms of how well they serve as guidelines. The combat maneuvers Grapple and Shove are fantastic examples of guidelines disguised as rules -- you can extrapolate the results of a lot of improvised actions from those two skill contests.