"Is there any protection inherent within MPLS that would prevent the oversubscription of smaller links (OC-3 to T1)? "
Within the provider, normally no.
"Is it possible my provider might limit traffic? "
Often only traffic such as DSCP EF is rate-limited, although other traffic can be out-of-contract and more likely to be dropped if there's congestion.
"How could I possibly write a QoS policy when 20% of the head-end is larger than 100% of the remote end? "
Often that's done by shaping the head-end to match the remote end's bandwidth.
"I'm trying to find the "nicest" solution in regarding to TCP/IP. I hear that policing is rather harsh on the windowing mechanism, and that shaping is a better option."
On many Cisco platforms/IOSs, default settings for both policing and shaping often do make policing more "harsh". However, both can and do drop packets, and with TCP windowing, drop management is an important but often overlooked aspect of QoS. (NB: BTW, [W]RED pursues drop managment, but just one aspect.)
"Would it be as simple as creating some type of nested QoS policy? "
It might be, but as Laurent has noted in this thread, and as other posters have noted in your other similar question you've posted in the WAN forum, often MPLS provides any-to-any. For example, consider three sites all with T1 but two attempt to send to one.
Again (as mentioned in other thread), if your logical topology is hub-and-spoke, then you can often manage the hub end to not overrun the spokes.
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