Of course magic items are destructible, that is without question, and because the enchantment is tied to the physical matter, the item ceases functioning, perhaps it can be mended, perhaps it will explode, who knows?
As with the walls: It puts something physical down. Wood, Earth, etc. So it is logical that people may try to harm it. Then there remains the question: how would a wall of air and a wall of fire put up next to each other, react? Would the air fan the flames, would the fire consume the air? What about a water wall? In any case: for the wood and the earth walls: the designers provided clear directions how to proceed and do damage to the wall (RMFRP, spell law, of Essence, p97). Also there is a system where the elements are either damaging each other or impervious.
As for the shield spell: its a plane of force...Pushy force that displaces the attack. Same as with magnetism pushing its opposite polarity out of the way. HOWever hard you strike with a reversed magnet, it won't budge. Except the shield is magic pushy force against everything. That's why I rule it is unbreakable, ow and the fact there aren't any hitpoints mentioned for breaking the shield.
Then there is the "golden rule" of RPG's: When physical force isn't solving your problems; you need to use MORE. I'm against this approach. I like my players thinking outside of the box and surprise me with something that isn't " bashing down the front door and charging in", much less; "Ow its enchanted so I bash it to take it out". So I'm not making an exception for the shield spell, but most spells can't be bashed to dispel.
As for the Arms master lists: I'm not ever going to allow anyone those, so its probably best to leave it at that, but I think you are confusing 2 professions with each other, since there were no lists dealing with breakable enchantments mentioned in RM companion 7.
As for the Illusions: They don't shatter (page 11, 28 and 98, Spell law of essence, RMFRP) They are there, except when you strike them and seem to pass through, you have detected that they might be illusions, or perhaps ghosts!
I hope to have shed some light on the matter,