I would put on a second floor now while you still can....
Or you could still go with the copper valley, make the pieces no longer than 8 feet long for expantion and contraction, also it would need a ridge/bend in the center to stop water from flowing up the other side of the valley.
On the edges you fold them over onto themselves about a 1/2 inch so if any water gets under the shingle it cannot flow off the copper. If you want to get crazy you can attach a tab to these beds and nail those to the roof so the metal can move and you dont penetrate your valley metal.
Last you always install some ice and water up the entire valley under the copper for that extra protection.
Copper prices are at a high now, however lasts and is nice to look at.
Even with the low pitch you will be ok. Shinled roofs leak as much as metal ones on low pitchs if things back up..that is why we have that human fly paper called ...ice and water.
I wish i knew how to draw a picture of thius....and ony if I kould spel....