Here is a wild thought, Mab or any other Fae will look you in the eye and tell you they cannot lie.  Now that may be true, in which case bargaining with them shouldn't be all that dangerous, but it is.

Of course that includes Molly now.

So why is that?  This is a wild theory that came to me while I was watching a special about King Tut..
Don't ask me why. But here it is, when you are bargaining with a Fae, it is like looking in a mirror.  We see our reflection, we think we are seeing truth, but the image is backwards..  So the Fae might indeed be telling the truth, but we lie to ourselvesas we hear it, or in another words it amounts to a lot of wishful thinking.  That's why it is so dangerous to bargain with the Fae...

This is oddly close to what Harry warned Maeve about. Once you can lie, you start lying to yourself.

When a Fae makes a promise in a bargain, he or she binds himself or herself to the promise.  As we saw in Summer Knight, Harry had bound himself to Lea for three promises in exchange for whatever she gave him to fight Justine.. However Mab was able to take over that contract, then promises were then moved to her.  In the case of Ethinu, Harry needed blood and had to say the right words to bind her, but she wasn't bound to Harry, she was bound to the prison.

Harry bound Ethniu. He beat her will to resist the binding. Not Demonreach. The prison is just where she is bound.

The Lady, Queen, and Mother mantles were already bound to behave under equal exchange. Harry did a favor for Winter, who were defending Chicago under their Accords, which the Fomor were attacking. Winter was obligated to return the favor. Harry demanded payment, they had no choice but to grant the favor or suffer the consequences, which we don't know what they would be. But, they would have to be serious.

When Mab acquired Harry's favor to Lea, we don't know how. Did Mab demand them as service from her vassal? Was it the price Mab extracted to cure Lea of Nemfection? Or did Mab trade a favor to Lea in exchange for Harry's favor? While Lea was incapacitated by being cured, Mab acted to fulfill her vassal's obligations. Just as when Harry was mostly dead Mab acted to fulfill his obligation to train Molly, transferring that obligation to Lea, who was already bound to fulfill Harry's obligation to train Molly as his Godmother.

Harry himself has a ton of obligations. And not just favors owed to others. He is owed favors and tends to forget them or brush them off. I'll be watching to see if he doesn't start taking those favors owed more seriously as the mantle takes hold.

Harry didn't "bind" Lady Molly to pay; that came as a surprise to him.

Go back and read Battle Ground, the part where Molly is walking around in a fireman's coat after the battle. Molly even said that Harry has bound a queen right after he makes the demand.

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)

The obligation against lying is Winter Law not particular to the Mantle, so Nemesis suborned Winter Law, breaking the restrictions of the Mantle would have meant Maeve becoming a undifferentiated lump of protoplasm. The same happened with the Mantleless Lea.

This fits in with my theory that Nameless became subject to Winter Law solely in order to destroy it, (he is a Demi-god of Discord, and a crooked Lawyer) his aiding Nemesis just one of the aspects of his attack. Without Winter Law then Winter would lose its coordination, massively undermining the defence of the Gates. Oh and Nameless is Cowl of course.

Slight correction. The prohibition against lying, or, more correctly, the requirement to tell the truth at all times is a fae requirement. Like the vulnerability to cold iron, all fae have this. Not just those under Winter Law or those that are part of Accorded nations.

It's a mortal cell phone. In SG she walks away and Harry sees her use it. In PT he summons her and she's using a cell phone that the mortal summoning magic lets the magic smoke out of.

Molly also thinks she can leave the circle, but when she presses a finger to it, the fingertip deforms like she's pressing it against glass.

Mab's instruction to Harry was to "Kill Molly Carpenter". Not, kill lady Molly. Or kill the Winter Lady.
Names, offices and mantles are incredibly important to the fae. Mab summoned Kringle, not Vadderung. Same person, two different names and offices. Harry bound Molly because the Warden of Demonreach did a favor for Winter, by enforcing the Accords and binding Ethniu. Winter was obligated to repay the favor and he called upon them to pay for the funerals and medical bills.
Harry Dresden, The Za Lord, The Wizard of Chicago, or the Winter Knight could not have bound Ethniu. Only the Warden of Demonreach could have bound her.
Name and mantles, kid. That's how they get you.