I got beat by a few people but since my post is done...
For armour, you don't stack the bonuses. You just take the highest value. I think there's an exception to that rule with one of the Endurance stunts that lets you have armour against blunt attacks. That's the only case I know of being able to stack armour.
Skill replacement...man...
I'm not sure where it specifically says you don't get bonuses. There's been a few threads where the developers said as much, I think. The non-beta book probably clarifies it. You definitely can not add bonuses to skills.
The exception would be creating an aspect. You could have a 6 shift item create 2 aspects which you can tag. Which, RAW, is acceptable but many GM's ban it or restrict it. The reason is at higher levels, you can have an enchanted item give you 4 aspects which you can tag willy nilly. Gaining 4 aspects for a single action is powerful. Not just the +2 tags, but also the re-rolls and the narrative power it can extend.
I think, in one game I'm playing, we ruled that you could create multiple aspects but you could only tag them one/turn for given roll...or something.
Given your example here, I don't think the rules really support duration separate from uses. While there isn't anything in the rules that prohibits duration, allowing it seems to allow a crafter to double dip . . . as your example above shows. You create a multiplicative effect turning what should be around 18-20 shifts of effect into 36 shifts of effect (3 shifts for 4 exchanges 3 times), and no stress spent.
The other thing to remember is that thaumatugry is meant to achieve a specific result/goal. Like making a curse, or answering a question with a divination, there's no need for duration...it either succeeds or it doesn't. It's the same for a weapons attack spell. Once you attack, the spell works (you hit) or it doesn't (you miss). But then it ends. So duration doesn't come into play. You can't make your weapons skill +10 for 5 exchanges with a single spell.