Metal is only an issue if they don't also change the smelt to bar ratios. As in, if I mine out a section of rock, within an iron vein. Big enough for a dwarf to comfortably stand and wide enough to house a bed (a 1x1 tile) I better damn well get more than a single bar.
As far as justifying it, it shouldn't be an issue. You could just say, "a block labelled '£' doesn't mean a solid cube of ore, but a cube of rock with enough ore distributed in it to produce 1 bar." So it can always 'make sense' no matter what is decided.
As far as gameplay goes, I like multiple bars to make weapons and armor. You can always just lower the MINERAL SCARCITY when you gen a new world in order to get 3x more ore than usual if 3 bars per breastplate seems too much. I find with default scarcity it's not very challenging to get lots of metal using the '1 bar = 1 item' bug as it is now. With the edits I made (closer to the 'old' stlye, 'intended style', whatever), I find myself actually using copper, since I don't immediately find enough iron to do everything.
Most forts, Properly adapted, don't need much of anything from caravans anyways, I use them mostly as a way to get rid of garbage.
To me, the mountainhomes are nothing but a landfill.
As per the bridges no longer being ultra invulnerable idea, I don't think it will matter, it'll take all of a half hour before we find something else to use as the go-to "I win" defense.
True, if bridges were made vulnerable, you could just use constructed walls instead. The issue is that bridges are a very inexpensive, simple, and fast way to control what areas are safe. If they were no longer indestructible, or could only be made of lots of iron, etc. then at least there'd be more trade off in making yourself totally safe (more cost if using metals, or more wasted time and risk of not being fast enough if you have to build/deconstruct walls to allow access).
I'm hoping tunneling (possibly swimming, to account for the aquifer defense), drilling, or catapulting enemies with the ability to dig through stone / destroy constructed walls and floors with siege engines come about, to truly eliminate the "Perfectly Safe Fortress." Sure, you could still have great defenses and tons of traps, but at least you wouldn't have the "pull a lever and become absolutely immune for eternity" mode we have now.