Expansion, Retreat & All Conflict states share the same state slot so only one can happen at once with Conflicts being priority 1. If you do see any faction in a Expansion plus conflict that's because Expansion state shows up for every system that faction is in so you can even see a Retreat Expansion.
Expansion and Retreat possibly share the same slot - no-one has managed to get a faction down to Retreat threshold while still in active Expansion, because it ends up in conflict in the meantime, so it's a little hard to say (though in practice doesn't matter)
Expansion and Conflict can definitely happen at the same time, because the expansion tax can be paid during the conflict lock to lead to very wide in-conflict influence separation (and the expansion tax is only paid from the source system). For example:
Explorers' Nation were clearly expanding 30 July - 5 August from Union, because it's the only system which got above 75% (25 July, 5 days pending) to cause the expansion ... and for further proof 5-6 August they take the expansion tax in Union (where the War is at the time) during the War state. Nothing else can cause a 15% one-day influence movement during a conflict lock.
Also to get a system from 75% down to the low 40's to high 30's in under two weeks needs many people flawlessly working BGS or the system has an extremely low population.
Certainly much easier with a lower population, but we may be considering different things to be "extremely low". There'd be plenty of time to spare in a million-population system, and that's around half of them.