jalview

SequencesInfo which is an individual object is doable, but I feel like it would be just a wrapper around a Map. What additional functionality would SequencesInfo have over a traditional Map?

I'm not very familiar with serialization in java, I know there are libraries that can serialize beans to json or xml, but what are your expectations in this case?

one class with a static factory method seems to be better than two classes and an external mutable object.

What's wrong with static classes? They are just like regular classes but within other class' namespace. That's non-static inner classes that are special.
About factory pattern, I think it would be an overkill for a class that is only used by SeqsetUtils' methods.

yes, I see that, but do you really need a nested static class ? in line with my general comments re encapsulating the Map<String,SequenceInfo> further: perhaps it might make sense to simply apply a factory pattern rather than have an internal bean that a generic map holds references to ?

(lets not get stuck on this though, the main thing is that the implementation works fine for now, though needs more work in order to save/restore sequenceInfo objects from Jalview project files)

Because it's instantiated in a static context e.g. uniquify and it does not need an instance of SeqsetUtils object to work.

we won't see this messages unless asserts are enabled. Cache.log.warn is perhaps better ?

This is waiting for us to define the "language" of edam classifiers that we are going to support. Currently we only define services by what operation they perform but there is plenty of room for fine grained tuning of operations based on other classifiers.