Network question (MPLS, ELANs etc..)

Trying to understand this network at work.  We have an MPLS connection with some of our remote sites.  Some of our other remote sites are coming through over the internet (IPSEC) and some are part of the Verizon ELAN.

My questions are as follows:

1. Is MPLS just a protocol that runs over a T1? What is the advantage of this, say.....over PPP on a T1?

2. Can ELANs terminate into a layer 3 switch? It looks like thats how the diagram is doing it. Is this recommended?

3.  Why are their two connections to the internet? (Comcast and Qwest). Is this common practice? When we surf the internet at work, our public IP shows up as being Comcast.  I'm thinking the Qwest one is soley used for VPN tunnels from remote sites

4. What kind of protocols and or physical connections does the MPLS line use? What about the ELAN?

Thanks
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