They are vulnerable to mortal workings. That's the reason iron and steel are extra-fatal to them. Even so far as paralyzing them when immortal fae are penetrated by it. Way back in Death Masks (?) Harry dumped reality powder of Lea, which was mostly made of depleted uranium powder, and it burned the crap out of her.
The reason why Molly can use high tech is that the mantle has either suspended her mortal magic, transformed it into fae magic, or somehow overriden her Murphyonic field. There is precedent. Harry and Molly used to sit inside a circle to have their magic contained if they wanted to watch a movie with their friends or family. Perhaps the mantle does the same thing, traps her magic inside her body somehow and applies a skin of the mantle's magic over her.
Equal exchange is part of balancing the scales. E.G. Butters removing the rebar from Mab's neck during the Battle of Chicago. He was not bound to do so by contract or treaty. Once she was back on her feet she offered him a favor to balance their scales. Even when Harry, as the Warden of Demonreach, bound Ethniu, he was then able to bind Lady Molly to pay for all the medical aid and funeral for Chicago. Because the Warden is not bound to Winter or a signatory of the Accords. By binding Ethniu, he put Winter in his debt. Ethniu was Winter and Accorded nation's enemy. Winter had to balance the scales with the Warden.
Mab passing through Harry's threshold was a matter of guest rite. Since she was not offering him any harm, she was behaving within guest rite and allowed to enter without permission. Though, as we saw in The Law from Harry's perspective, most of her power was probably left outside.
The Svartalves made Harry's upgraded ring in his workshop. Which was copper, silver and iron, combined into one ring. He had to pay a premium for them to work with iron. I thought he'd lost it in Changes, but it was still there in PT. But, per WOJ, not everything native to the Never Never is vulnerable to iron. (At least not vulnerable beyond the usual, like being cut by a steel sword)