Hello Harrison,
The L1 Compact is a great little system, but I understand your feedback woes. My wife and I perform as a duo with a pair L1 Model 1's, and we separate the vocals -- one mic to one L1 (the original design concept for the L1 systems.) I purchased an L1 Compact and T1 mixer for smaller, intimate gigs. We immediately faced feedback problems with two mics into one system. We had to be careful to keep our faces in front of the mics, shielding them from the array, and keep them pointed away as well.
Since the Compact's line array is only 6 speakers vs. the 24 in the Model 1, they have to be pushed just a wee bit harder. With two mics, the chance of feedback is not twice as much, but nearly four times as much. Our solution was to get another Compact (yes, I know that's not a solution for everyone) and we've never had a problem since. A single Compact can get loud, but with multiple mics it's a challenge to do so without feedback, and it is not perfect -- it has its limits.
Just to cover a few basics: Be sure to gain-stage your mics and instruments in your T4S first (there's a great video here, in the Bose Professional Portable PA Encyclopedia.) Pan all your channels right, and connect only the right main out to Channel 2 of your Compact. You'll need to experiment with different mic presets for your Telefunken mics -- I'd probably try a Shure Beta 58a preset first and go from there. (There's no harm in trying any preset -- even instrument ones!) Then zEQ to taste. Then set your L1 Compact to about the 4 mark, bring your T4S master up between the 2 and 3 marks, and mix your sound with the channel volumes. Use the T4S master to adjust the overall volume. If you hit red on the Compact's channel 2 light, then back off the T4s master. Test raising the Compact and lowering the T4S and vice-versa to find an ideal setting -- it's a dance between the two.
System placement will always make a difference. Behind you is ideal, as you can then hear exactly what the audience hears, but to the side or even slightly ahead of you will work. You will still be able to hear the system clearly. The "throw" of the system is surprising, especially in the vocal range, so you might not need to be as loud as you think. You'll need to get constructive, objective opinions from someone in the venue.
You may also experiment with Noise Gate settings in the T4S, to "open" the mic only when singing directly into it. (I know there have been several forum discussions regarding use of the Noise Gate, but I have no direct familiarity with it.)
Anyway, that's my experience. I hope others will chime in with theirs as well.
Does that help?