The answer to your first question is yes. All hose connections in a sanitation system should be double clamped, with screws 180 degrees apart.
The Jabsco instructions are a bit incomplete and also assume a downhill run from the toilet to the tank or thru-hull, allowing gravity to take over as soon as you've emptied the bowl. But when there's a vented loop in the discharge line--typically 2-3' above the bowl on a sailboat, the flush has to make it all the way up and over it...anything that doesn't runs back downhill to the bowl as all liquids are prone to do. A brand new joker valve (a rubber cup-shaped thingy with "lips" that has a slit in it in the toilet discharge fitting) may block it out of the bowl temporarily, but as the toilet is used it stretches the slit and fairly soon it begins to allow slow seepage. As it becomes more worn and the slit gradually becomes a hole, it ceases to block or even slow anything down. From your description, I'd say that's about the condition yours is in.
So there are two things you need to do: Replace the joker valve (you should do that annually)...and after you've done that, learn how to use the dry mode to do more than just empty the bowl. The only difference that pumping a zillion times in the wet mode before switching dry makes is whether what's running back to the toilet is clean or dirty...and you fill up your holding tank at least twice as fast as you need to.
Most people don't realize that any manual toilet that's working anywhere NEAR factory spec can move bowl contents up to 4' vertically IN THE DRY MODE. So use the DRY mode to push the flush over the top of the loop (may require a learning
curve to figure out how many pumps that takes)...switch to wet for a few pumps to bring in some rinse water...and use the dry mode to push that over the top of the loop. You'll still have a cupful or maybe even less of runback, but it'll be clean water instead of waste.
If you need help in more detail than is practical in open forum, you're welcome to contact me directly.
"If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein