Don't use Self Levelling, that's a different product with a different purpose. Just use cheap sand mix or mortar mix that comes in a bag, mix it with water so that it will stay in a pile when you throw it on the floor. The shower base will then squish the cement into place while filling any voids under the shower floor pan.
The plastic or roofing felt is to keep the moisture in the cement so it can cure properly. Future movement of the t&g from atmospheric changes won't effect the cement and try to crumble it over time if the plywood is there for stability.
Any raising of the foundation can certainly have an effect on the floor positions and elevations.