More than just two things...
Flush water filling the bowl: All manual marine toilets have a lever or knob that operates a "wet/dry" valve in the pump...in the wet mode, pumping brings in flush water and pushes bowl contents out when you pump...in the dry mode, the flow of flush water is blocked. That lever should always be left in the "dry" position except when using the toilet. If the bowl fills when the lever is in the dry mode, that valve has failed and must be replaced (depending on the make/model/age of your toilet, you may be better off replacing the whole toilet). But whether or not that valve has failed, all manual toilets that are below the waterline should have a vented loop installed in the head intake (something that's the owner's responsibility, not the toilet mfr's) to prevent flush water from rising in the bowl if the wet/dry valve is accidentally left open OR in the event the valve fails. The loop should be at least 8-12" above waterline at any angle of heel, and should go between the pump and the bowl, NOT between the thru-hull and the pump...if you put it between the thru-hull and the pump, it will prevent the pump from priming. However, while a loop will prevent flush wate from filling the bowl while the boat is sitting, it will not keep the bowl from filling while you're underway, heeling on the same tack as the head intake thru-hull if the wet/dry valve is left open or has failed.Whether there's a vented loop in the line or not, be sure close ALL seacocks before leaving the boat...open thru-hulls while no one is aboard is the #1 cause of boats sinking in their slips.Waste backing up from the tank could be due to several reasons:If it happens when the boat is sitting, your tank vent may be blocked, causing the tank to become pressurized. Tank vents must be kept clear.If it only happens the inlet fitting on the tank is outboard on the tank, when you're heeled even a half-full tank can spill into the head discharge line and run back toward the toilet. Rob mentioned the joker valve--a one way valve in the head discharge fitting (not ahead of the pump, but coming OUT of it)...it's rubber "cup" that has a slit and "lips", and should be replaced at least every two years because over time, waste going through it stretches the slit till it can no longer close. However, joker valves are only supposed to block a flood, not slow seepage...so even a new one won't prevent some waste from getting into the bowl if you're on the same tack for hours. So in addition to replacing the joker valve, there are two ways to solve the problem:1. A loop in the head discharge line that's at least 4-6" above waterline at maximum heel...it doesn't have to be a vented loop, just a "hill" higher than waste from the tank can climb over.2. Move the inlet fitting (not nearly as hard to do as you might think) to the other side of the tank--away from the hull, to side closest the centerline of the boat so waste runs away from it when you're heeled. The vent fitting on the tank should also be away from the hull, toward the centerline of the boat, to prevent waste from spilling out the vent--which can result in a clogged vent--while heeled.As or whether to rebuild or replace your toilet...that depends on the make/model/age...'cuz the rebuild kit can cost you more than half the price of some toilets and still won't solve all your problems. I suggest that all three of you check out the link below to learn how marine toilets work, and how to prevent 99% of problems.