Sounds like you are ready to invest in your own PA system!
Seriously though, if you follow some basic steps, you should be able to get close to the sound you want, provided the house PA system is up to the task. First, you are correct in your observation of not using a mic input for a line level device. The mic input is much to sensitive, and has the wrong impedance, making it easy to distort the sound when a line level input is fed to it. Second, though obvious, make sure you are not sending any signal to your effects channel on input 4 (your vocalizer return) This would give you a built in feedback loop.
Third, tackle the feedback problem. Feedback is effected by speaker placement and frequency settings on your sound system. If at all possible, you want your PA speakers in front of your microphones The worst setup would be to have your PA system behind you, with the speakers facing directly into the microphones. The second worst setup is to have the spakers above you, facing down. This is how many Hotel banquet rooms have their PA systems set up. It is almost imperitive to use a stand alone system if the house system is built into the ceiling.
Over amplified high frequencies are much more prone to feedback than mid or low tones. Use your mixer's equalizer controls to cut the high frequencies a bit, and to boost the mid range.
The third thing is set up your sound sytem so that it sounds good to the audience, not only to you in the headphones. I use a disk in my PSR 730 to set up the PA system for the house I am playing. It allows me to get out on the floor and hear how the mix will sound to the audience. I also use a tape recorder (boom box) with some good vocal tracks on it to get the mic mix close to where I want it, then I check it with some tests with my performers. Pay attention to your ears, don't worry about where you end up having to set the knobs. If you end up with the highs cut way back on your mixer, and a heavy boost to the mid range, and a flat bass setting, so be it. IF IT SOUNDS GOOD, IT IS GOOD. Every venue you play will be different, don't be afraid to twist the knobs. This is where it really pays to be able to get into the hall early and set up, there is no substitute for actually being out on the floor where you audience will be and seeing how your setup sounds.
Hope this helps,
Dave Waldman