1. The Mk. I carriages were limited to 100mph. Actually, they were originally limited to 90mph until the bogies were upgraded, although I leave that out as it would overcomplicate things somewhat. It is the Mk. II carriages that were capable of 110mph (and, of the things that could haul them, only the class 87 could achieve that speed before the 1980s, although the game also has a diesel locomotive, based on a never produced prototype, that can also travel at 110mph). No Mk. II mail carriages were ever produced (nor, indeed, any mail carriages after the Mk. Is, aside from a special multiple unit train). In reality, there were no 110mph mail trains: mail just did not need to go that fast.
2. Coverage is not the only incentive to have more frequent stops. The time taken walking to the stop, in the case of passengers or, in the case of mail, walking to the postbox carrying one's letters, is computed as part of the journey time. The nearer the stop/postbox to the origin building, the lower the journey time. Passengers may not travel at all if the journey time is too long: this does not affect mail, but a rival company's mailbox at closer quarters will be able to take your company's traffic, as the shortest total journey time is what the mail will use to decide which route to take, including from which postbox to start. This is a far better and more realistic mechanism than having a small coverage radius (in reality, people used to walk miles when there was no alternative), although if you prefer a smaller coverage radius, you are free to change it in simuconf.tab.
3. The idea of requiring mail to be sorted before it is delivered has occurred to me, although this would be complex to implement and will have to wait for the far future when a very long list of higher priority features has been implemented (unless somebody is willing and able to code it sooner).
4. The costs generally are not balanced yet: balancing anomalies should be corrected when the balancing exercise is undertaken, but this will require major feature changes (very slowly in progress) to be done first so that the balancing process can produce a workable balance and is not trying to hit a moving target.