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First, be aware of, on 3560/3750, some stats refer to the four egress queues as 0..3, others 1..4, which is great for making it even more confusing.
q1: The dscp egress table shows you the count of packets with the different ToS markings. So, for example, your:
dscp: outgoing
------------------------------- 0 - 4 : 2351172567 870 814120 0 5
Provides the count of egress packets with ToS markings 0..4, zero being DSCP BE. (A lot of those, which is often the case.)
If you want to know what queue and WTD a ToS marking goes to, you look at your Dscp-outputq-threshold map. If we want to see where the BE packets go, we look for a DSCP value of zero, e.g.:
#sh mls qos maps dscp-output-q
Dscp-outputq-threshold map:
d1 :d2 0 1 2 3 4 5 6 7 8 9
------------------------------------------------------------
0 : 04-03 04-03 04-03 04-03 04-03 04-03 04-03 04-03 04-01 04-02
In the above, they go to queue 4, WTD 3.
Your egress stats, shows whether packets have been enqueued or dropped. Remembering my first note, the stats that apply to those BE packets are:
output queues enqueued:
queue: threshold1 threshold2 threshold3
-----------------------------------------------
queue 0: 0 0 191512419
queue 1: 3921076 594772 52868524
queue 2: 0 0 1504645786
queue 3: 664882 1040426034 2353605945
output queues dropped:
queue: threshold1 threshold2 threshold3
-----------------------------------------------
queue 0: 0 0 0
queue 1: 0 0 0
queue 2: 0 0 7772375
queue 3: 1523 1023494 132058
Remember, though, other packets may map to the same queue and WTD, so these stats may, or may not, just apply to BE tagged packets.
q2: I don't know how they decided on using 3100; probably they just wanted something a little under max. What they chose may be just to make an example.
Maximum value varies per IOS version. Older versions only supported a max of 400, later versions support a max of 3200 (if I recall correctly).
I'm unsure what your "scenario" is.
q3: It depends on what you're trying to accomplish and the nature of the traffic flowing thru your device.
As Daniel has already noted, 3750s are well known for egress interfaces dropping packets. This because they don't have large buffer resources (2 MB per 24 copper ports or per uplink ports), and their default buffer settings, especially when QoS in enabled, might be far from optimal for your traffic.
Personally, I have had great success enabling QoS and setting the buffers to used a shared buffer approach (more like the earlier 35xx switches). However, I cannot say whether such an approach would work for you.
