Maybe not. Maybe once the process of removing Nemesis has been started, it cannot be halted, or she risks Nemesis entering her mind.
I kinda doubt it, but it's possible. Mab's "cure" seems to be more like a mental wall of ice blocking Nemesis from entering whatever backdoor it made in Lea's mind.
As a different thought along those lines, perhaps Mab made a masterful counterstroke with Lea. If Nemesis has a finite amount of hosts it can bounce between, keeping Lea alive but unable to be controlled might deprive Nemesis of a host body it could otherwise have. Say the Arctis Tor attack was an assassination on Lea gone wrong. With Lea on ice, Mab should have been at the Outer Gates. Perfect time to make a ballsy move in her empty palace. Except, nope there she is. Time to leave as quickly as possible? Mab might have chased them, but they're not big enough fish to deserve her personal attention. Especially if it might have been a feint and more attackers would sneak in with Arctis Tor actually empty.
That kind of idea would have the right caliber of enemy to kill Lea (hence burn Arctis Tor) but not worry Mab.