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Question 1: If there were 100 1Gbps links, could I use a Catalyst 3850 stack as an aggregation device? Its spec says its stacking bandwidth is 480Gbps.
480 is the total bandwidth capacity for 9 switches stacked. For 100 1Gig links, you don't need 9 switches. You can use 3 or 4 switches and get line rate per port. Now, if your uplinks from your aggregate switches is only 10 or 10Gig, than that is your limitation.
My understanding of StackWise-480 is the 480 Gbps represents 480 Gbps of duplex bandwidth, across a pair of ring ports. Or, if a "ordinary" Ethernet port, we would consider each ring port 120 Gbps.
StackWise-480, like StackWisePlus before it, supports spatial reuse, so as you add stack members, you increase aggregate ring bandwidth. However, as stack member traffic may need to transit multiple ring segments, you don't want to assume all the aggregate bandwidth is available. Cisco says "The spatial-reuse technology enables multipath parallel switching across each stack ring to double the throughput." Cisco appears to be assuming traffic will only need (on average) to transit half the stack members.
From just a bandwidth view, two such stack ring ports should handle 100 gig links but other considerations might/should be considered whether these switches are suitable for the distribution role.
Question 2: 16 10Gbps links connect to a c6500, could I use four 4-port 10GE line card to achieve 1:1 oversubscription?
Yes, you would need to put the ports in performance mode.
BTW, don't believe performance mode applies to 6704s. (I.e. don't believe it can be configured, nor should it be needed.)
I recall only the (discontinued) 6702 offered true wire-rate (with sup720s), but also (vaguely) recall the 6704 doesn't offer 100% bandwidth needed, but it comes very close. Also when using the 6704 and especially all four 10g ports, CFC and/or DFC3 capacity PPS might be a bottleneck; DFC4 has the additional PPS needed, I believe. (Also with 6704s, you'll want whole chassis in fabric mode.)
