I cannot get the top w/ gasket back on a sanibest after servicing the macerator

You should be able to purchase this at any local hardware store.
Lowes, Menards , or Home Depot. They may have to order it but I am sure that if you take the old parts they can make you a replacement if they do not have the direct replacement hose there.

http://www.saniflo.com/homeowners/sfa-product-line-for-homeowners


Try this site. Bottom of page has Macerators.

The float ball is hung up on debris and need cleaning or service. Use rubber gloves!

Not any more than any other 1.6 gallon flush tiolet.

80 percent full

Bail out as much as you can through the toilet bowl. Unfortunately this is a messy job regardless and you will still need newspaper on hand.
If you lower the water level below that of the rubber gasket connecting it to the toilet though, you will be able to undo the jubilee clip holding this and slide the unit away from the toilet, hopefully without spilling much waste water.
If you can dislodge the blockage from this angle, great. If not, you might also have to take the lid off by depressing the plastic clips on each side and accessing the chamber from the top.

I had the same issue before. Definitely has to do with the float switch. Could be that a piece of solid is stuck at the bottom of the float switch or the membrane (black rubber piece at bottom of flat switch) may be warped/deteriorated and will need to be replaced.

The float switch will be the problem.
We do not replace the switch, just change the unit as labour to strip and replace switch makes it un economical.

need to replace the flapper

depends on how you use the unit. the macerator chews up paper and other unmentionables.I would suggest you use it as one unit.