Iolanthe's spear sweep is possible. If you thought it was impossible from the beginning, which was what... two posts ago, why didn't you tell me about it then?

Iolanthe lowered her arm and the spear initially for a sweep, just as you assume. The power isn't coming just from the slashing motion, however, but a secondary stabbing motion she's making the minute she was able to sneak Hyperion underneath Sigurd's shield, breaching his guard. There's absolutely no way I see Sigurd blocking the spear now, since he swung his shield off idle to the side, and relied too much on his attack forcing Iolanthe's hand to retract. She'd have only needed a 1-2ft gap in the first place to make this happen. Remember, she's also slightly ducking, and the whole spear is 6ft in length! The sweep initially aimed at his feet would've been weak, yes, however it is being re-adjusted, and aimed to stab in-between his legs, thus being given new momentum, new kinetic force, and mortal danger.

There's a chance the blade's end might slash the left side of his left leg first, however since you stated Sigurd is now leading with his right, I assumed both of his legs are in the path of danger, and his left leg might actually be a whole 6ft away. Although it's a stabbing motion, she's relying on the edge of her blade, making the damage it'd deal a 'slicing' wound.

Also, the heat from her spear is strong enough to melt metal. Leather and fur might ignite, or be burned to ash with ease.