Only rarely. The majority of series ratings allow the upstream device to be mounted remotely from the downstream one(s).
Did you contact the designer?
I was not thinking of a situation where the upstream device is prohibited from being remote mounted, rather I was thinking of a situation where, either as a cost saving measure or just because its what you have, one needs to take advantage of the impedance of the feeder. For example lets say I have (or need) a main, distribution, and branch setup, lets call them A, B, D respectively. Yes I am skipping C for now. Lets say AFC at A is 75K, B is also 75K (they are in the same location), D is 20k. So we have some options here. A three tier rating would of course work, but I believe they are somewhat limited in choices. I am also going to assume that generally the most economical setup is to be able to use standard 10k branches. So we could go with fully rating breaker B, and use a series rating for D. However I see two potential problems with this. One is cost of having to use 75k distribution breakers. Second, if the distribution breakers (B) are over 250 Amp frame, it is unlikely you will find a series rating that gets you down to 10k branches. So another alternative, which gets in where I could see using a MB in the branch panel, is to series rate B to A which will allow the use of less expensive AIC distribution breakers. Then I could use 22kaic MB in the branch board (which is fully rated for the AFC there) which would allow a series rating with the 10k branches.
Yes, the scenario you mentioned is possible, but not likely. The size of the breaker is probably a concern wether it is upstream, as a feeder, or local, as a main.