When I fill a carto, I hold it at a 45 degree angle (slanted, not straight up and down) and drip while rotating it. This allows the juice time to saturate down before I get back to put a drop down in the same spot. Not using this method makes it easy to think the carto is full because it is pooling at the top of the filler material even though it is not full, or to drip it straight down the hole without saturating the filler.
If the juice is leaking out of the middle hole, you are either 1) overfilling it by alot or 2) dripping it down the hole rather than into the filler material.
Some really sugary flavors kill a carto quickly. I had a small bottle of cotton candy that I let sit in my storage for a few months with others I'd bought at the same time...the CC turned dark brown and thick-like it carmelized or something-while the rest remained the same as they were. It has to be something in some of these flavorings that changes in composition with heat maybe? I dunno, but many people report that the sugary flavors are killers.
Also, the higher the VG content, the faster they can clog and quit on you.
I don't see how you could kill a carto by condom filling it, but I'm certainly not an expert. Regarding flavors-you can mix whatever you want. But you won't know the true flavor of a juice unless you use a clean carto or drip.
Cartos CAN be cleaned. I blow them out, then boil water in a pan and remove it from heat and soak cartos for about 10 minutes, then rinse them, blow as much water out as I can, and let dry overnight. If you are using the same juice in them, this method works great. If you want to use a different juice in them, sometimes there will still be a lingering taste of whatever was in them before. Some flavors are worse than others-my mint juices rinse totally clean and you can't taste any afterwards-but vanilla just will NOT no matter what.
Not sure what is burning your throat. The only time I've ever heard of that is from allergic type reactions. A friend of mine can't vape anything with cinnamon in it, and it is in some juices that you wouldn't think it would be. Rarely people are allergic or sensitive to the base and have to try varying the PG/VG contents until they get a mix that doesn't bother them.