I see you understood me wrong:
as for the items you mentioned: the AT of the tin foil full plate would be: AT 1 (tin foil is worse than canvas), -20-50+5 = -65 DB; ie. not really viable.
For me it is logical that bonuses stack since it does in real life: that's why, as a crafter myself; I work with the best gear available: Blue Bosch, Festool and Dewalt (well not the cheap stuff but they have some really nice tools). So Quality, craftsmanship, material, and any construction bonuses stack with a magical bonus. This magical bonus is just an addition of +5 (%) because for high bonuses you need better materials as a base: so high steel is +10, magical high steel is +15 and you need a Enchant inorganic III to make it so.
Mithril is one of the few magical metals in existence (being a special creation of Eru as a present for the Dwarves and the Numenoreans.) so it is at the basis of creating magical materials, perhaps except Lean, that special tree resin (forgot the name) and Mallorn. Ithilnaur, Anornaur, Ithildin, Eog and Galvorn all utilize some Mithril to attain their magical bonus.
I'm not telling you how it is, I'm just explaining how it works in my campaigns, of course.
I think you got mixed up in the description of how I let items stack on a character. It is a direct grab from the D&D 3.5 rules, except I have to explain it to RM users, with RM bonuses. The issue here is that my players (most being Ph.D.'s) have the player-from-hell tendency of taking a ruling and then bending it backwards and exploiting it. So wearing multiple DB items, only 3 can used, which would usually be the best ones: magical deflection DB, magical shield DB and Forcefield DB (usually magical). If I would take the Grumpy approach (which he explained some time ago: letting every item work, except the first item was full bonus, second was at 90% bonus, 3rd at 80%) you bet that my more crafty PC's would gladly buy up ALL available DB enhancements, and ask if a rapier welded to a pouldron was also a way of wielding it.
Hope I cleared up some chaff from my rules;
Joeri