1: If you're only seeing slug flow once per 2 months, you may be normally operating at only liquid or gas capacity. Again, I would verify that you calculated your LOF at maximum ρv², and not some averaged ρv² your process invented for a slug case.

Fatigue need not be limited to "high cycle" applications, but I agree that 180 cycles in 25 years would be an abnormal fatigue analysis. If you instead expect, say, 50 slugs for that "once" every 2 months scenario, you definitely would be worthy of fatigue analysis. (Typical cut-off is 5,000-7,000 cycles, but is really up to the end user.)

2: Nothing to add

3: Nothing to add