1 - It depends on the cartomizer - some cartomizers are loosely structured inside and don't stand up well to repeated refilling. I prefer cartomizers with a defined center air channel.
2 - If depends on the juice you use. Dark, cloudy or opaque juice can often drop flavoring sediment, clogging the stuffing and airhole and baking onto the coil. And thick juices, since they sort of impede wicking and airflow already, can lead to easier singeing and shorten the useful life of a carto. I use thin (like 80PG/20VG) clear juices.
3 - It depends on how well you keep the carto damp. If you let a carto get too low on liquid, especially with hotter cartos like the LR ones 450mah+ batteries can use, you will singe the carto and add a bitter burnt taste
I do not do "refills". I keep damp. I can get 7-10 days use out of each carto. Since I carry 2 devices, each with a carto, that is 2-3 weeks for me. I can usually run 15-20ml of juice through carto before switching to a new one.