This is a good start on all things Murphy/Mab. I'm going to search it for what you ask. The thread has my comments addressing many issues and links to supported reference materials that delve into Mab, Winter, and such.
http://www.jimbutcheronline.com/bb/index.php/topic,50304.msg2291717.html#msg2291717Serack's "When Winter took over at the Outer Gates"
http://www.jimbutcheronline.com/bb/index.php/topic,48717.msg2258880.html#msg2258880Mab's history.
http://www.jimbutcheronline.com/bb/index.php/topic,50304.msg2292401.html#msg2292401Fitting Murphy into a Mab ascension via Lady mantle.
http://www.jimbutcheronline.com/bb/index.php/topic,50304.msg2293072.html#msg2293072So, Murphy would be Winter Lady, for how long I don't know (WOJ our Mab as a Winter Lady chose Lea to be her handmaiden). Her appearance would have to already begun to shift. And, then some sudden set of events that aligned with the last time things got awful in the wizarding world, it ended with at least both Mab and Titania dying, who else we don't know, which allowed our Mab, our Titania to assume the mantle of Queen roughly around the time of the Battle of Hastings. Around that time, Mab and Titania stop speaking. Give us 1000+ years and I don't think we can assume we would be able to identify Murphy that had been Mab for 1000+ years is our Murphy. They'd look totally different. So, that is one thing my theory counts on, a mask for Queen Mab that camouflages her identity, which will stun us when it is revealed. This though is complicated further by requiring Murphy's children to have different genetics due to this change. That could be supported by faerie genetics though with changelings looking sorta like their faerie parents.
In short:
1) We don't know who the prior Mab was to our Mab, we do know however that she was killed around 1066 along with Titania. This event triggered our Mab and our Titania's ascension. It also led them to be very wary about Harry, whether because he is a starborne or other reason, they have drawn a similarity between him and whoever caused an event then that led to significant disruption in faerie.
2) That queen wasn't likely the original. We do know though that the original set of queens, (mothers, queens, and ladies) participated in an rite to establish their original base of power using the stone table.
3) We don't know who the prior Winter Lady was. Should Murphy have indeed inserted herself as Lady, that Lady would need to die and at a place of a conjunction like at the stone table or on Halloween such that the mantle would be passed and the immortal Lady be not so immortal. This wouldn't require Murphy to actually kill the Lady, that would still be possible, but she'd just need to be close in proximity and higher up on the priority list for the mantle than anyone else.
4) Not likely, the Winter Lady's are expendable and change fairly often.
5) Unknown. We don't know anything much about the prior Mab and her children. We do know that our Mab finally gets two weapons in her hands: Morgana LeFey's dagger and Medea's athame. This may hint that one or both may have become Winter Queens or Ladies at one time or another.