As per a thread I created on Spigot (https://www.spigotmc.org/threads/391433/), it appears the API features in PotionMeta are incomplete. According to the Javadoc (https://hub.spigotmc.org/javadocs/spigot/org/bukkit/inventory/meta/PotionMeta.html), you can only get whether the potion effect is extended or upgraded, and using the BasePotionData, you can get the type. The custom effects list is also empty unless you add to it using the API, but that's not helpful when trying to get the potion data.

It appears it is virtually impossible to get the duration or amplifier of potions on 1.10+ (using the deprecated Potion class does not function on 1.10+).

Any other questions please let me know.