Hello Winston,
2) yes P and PE nodes should be contiguous in order to build an end to end MPLS LSPs to be used for MPLS services, forms of MPLS over GRE are possible to pass through an IP only network portion, but usually this may lead to MTU problem and not all platforms support this.
You may want to consider a Carrier supporting Carrier service where you deploy your own VPNs on top of a VPN provided by the service provider.
1) Usually MPLS VPNs are used to remove the need of multiple WAN services because they are scalable and they provide bandwidth over it.
You can have IPSec access to an MPLS VPN or DSL access to MPLS VPN in order to provide services to remote users.
Using an MPLS VPN only for remote users it is not common: once you gest accustomed to MPLS VPN and the advantages it can provide you will probably go on and you will migrate all the network over it.
If you work for a company with very high security requirements you may want to use MPLS VPN just to build your own IPSec over GRE mesh between sites, for example you could use DMVPN of GET VPN for this.
Some advantages are lost but if encryption is required this is a feasible way to do it.
Hope to help
Giuseppe
