Rehearsing and performing live? How to monitor and avoid feedback?

This is a big question. You will be using different speakers for playing live, than studio (of course). For playing live, there are two primary paths often followed:

1) Most common. The "main" PA speakers are placed well in front of the performers, which the audience hear at higher volumes. The performers depend on either small monitor speakers (which can carry different mixes), or wear in-ear monitors. So, you can hear the monitor audio at lower volumes, but still have to be careful of mic feedback. If using speaker monitors, they are usually placed on the floor ("floor monitors"), in front of the performer, facing towards the ears. If using mics with the right pattern, feedback is minimized.

2) Bose L1 systems, which act as the main PA speakers AND the performer monitors. I use these, and really like 'em, but they are limited in application. When they suit your needs, they work great.

Some of your questions go beyond what can be covered in a MainStage forum, so you might want to start searching online for info on PAs and monitor systems, or live sound. Plenty of online info beyond what I could cover here.

In general, if you are using a Duet, you will only have two outputs from MainStage. So, unless you are using some external mixer, you could potentially only get two mono mixes out (L for one channel, R for the other) .... one for the PA, and a then a single monitor mix. If you do a stereo mix, your monitor mix would be the same as the mains.

The reason a group might want to have differenttheir mixes in different monitors (a much more elaborate setup), for example the drummer may want to hear more bass, less keyboard. Singer may want more guitar or keyboard, less bass. Etc.

In ear monitors completely eliminate the worry of monitor feedback. The only downside is some folks don't like 'em. They take awhile to get used to, especially single ear, but they work well and can save your hearing if used properly. Another drawback is wiring unless you go wireless (more expensive). Another big advantage is, less to carry (monitors can be more to lug around). Read reviews on someplace like Sweetwater for different models.

The size of your venues is also important. If the places are small enough, you could get by very well with a pair of powered studio speakers. I do quite regularly. I rarely need more than 200w for solo & band gigs. Position these carefully, & you won't need any monitors.

And of course this also depends on the output and size of your studio monitors, especially for low frequencies. If at all possible, don't risk using your precious studio monitors for live performance if you play out a lot. If you do/must use your studio monitors, be REALLY sure you are careful not to overdrive them and blow a speaker. Expensive! Also, you can be fooled by how much sound a live room full of adoring fans will soak up/require versus the same monitors when used in a soundproofed studio settings.