ThePonyTheorist 
[@Incursor](/images/2846594#comment_10058627)
Magic can pass through solids, yes - but it has also shown itself to be capable of teleportation. The reflection still works if the substance is exceptionally bouncy when compressed, and can do so best on low-friction (e.g. reflective) surfaces. Another explanation could be that the gas follows some form of electromagnetic radiation, I suppose. Sci-twi does state that magic is electromagnetic, but even Sunset states that it operates differently beyond the portal. The nature of the substance may be entirely altered.
Additionally - look at what Discord creates. He acts as if he's disorderly, but really he makes very high-order constructs which are just in a bizzare context or act as living puns. Like singing gin-sing tea, flying pigs and the like - these are low-entropy systems, not high entropy systems.
(also, if i'm correct in saying magic is ionically charged, it should be more likely to disperse - so it really needs this 'negative entropy' effect to even form the balls that we see. Therefore, proving that the negative entropy theory is incorrect would effectively destroy the 'magic is ionic gas' theory)
It's possible that changelings had to steal love in order to power themselves - but it's also possible that they intrinsically resist sending their 'emotional energy' (for lack of a better word) into the air, so they can get a net positive by feeding on a pony etc. Thorax's system likely pools magical energy between every changeling, and then each changeling's friendship feeds the whole.
Finally, the last part of my theory actually resolves all of your questions. I think that runes are responsible for ALL spellcasting. Unicorn horns? They have an internal region with high surface-area that changes color to etch extraordinarily complex runes on its surface extremely quickly. Pegasus wings? The bones themselves are COVERED in a thin layer of runes, commanding motion and all sorts - same with their hooves and possibly other parts of their bodies commanding their magic to disperse over clouds they contact to make a semi-solid structure. Earth ponies? Runes are etched into their muscles, and actively aid in their physical motions to magnify their strength to an obscene degree (AJ's apple carts weigh like 100kg each, and she can drag 3 like it's nothing. Earth ponies stronk is an understatement).
There's one image where Rainbow Dash is electrocuted, and you can clearly see a strange bone in her body, in fact. See https://derpibooru.org/images/1172591?q=screencap%2C+rainbow+dash%2C+electrocution
Magic can pass through solids, yes - but it has also shown itself to be capable of teleportation. The reflection still works if the substance is exceptionally bouncy when compressed, and can do so best on low-friction (e.g. reflective) surfaces. Another explanation could be that the gas follows some form of electromagnetic radiation, I suppose. Sci-twi does state that magic is electromagnetic, but even Sunset states that it operates differently beyond the portal. The nature of the substance may be entirely altered.
Additionally - look at what Discord creates. He acts as if he's disorderly, but really he makes very high-order constructs which are just in a bizzare context or act as living puns. Like singing gin-sing tea, flying pigs and the like - these are low-entropy systems, not high entropy systems.
(also, if i'm correct in saying magic is ionically charged, it should be more likely to disperse - so it really needs this 'negative entropy' effect to even form the balls that we see. Therefore, proving that the negative entropy theory is incorrect would effectively destroy the 'magic is ionic gas' theory)
It's possible that changelings had to steal love in order to power themselves - but it's also possible that they intrinsically resist sending their 'emotional energy' (for lack of a better word) into the air, so they can get a net positive by feeding on a pony etc. Thorax's system likely pools magical energy between every changeling, and then each changeling's friendship feeds the whole.
Finally, the last part of my theory actually resolves all of your questions. I think that runes are responsible for ALL spellcasting. Unicorn horns? They have an internal region with high surface-area that changes color to etch extraordinarily complex runes on its surface extremely quickly. Pegasus wings? The bones themselves are COVERED in a thin layer of runes, commanding motion and all sorts - same with their hooves and possibly other parts of their bodies commanding their magic to disperse over clouds they contact to make a semi-solid structure. Earth ponies? Runes are etched into their muscles, and actively aid in their physical motions to magnify their strength to an obscene degree (AJ's apple carts weigh like 100kg each, and she can drag 3 like it's nothing. Earth ponies stronk is an understatement).
There's one image where Rainbow Dash is electrocuted, and you can clearly see a strange bone in her body, in fact. See https://derpibooru.org/images/1172591?q=screencap