Positioning is not the only thing that'll help you out here. I've set up stages with speakers behind the performers, facing out into the room, it can be done.
There's a lot of factors which can cause feedback, including the resonant frequency in the room, quality of your equipment, etc. So for any answer to this question you'll have to experiment a little in the venue to find the right solution for you?
Have you tried playing more quietly? I know it's a radical suggestion, but it really can help. This solution may require ego-checking on the part of the band.
You say you're in a small room? How small? Do you really need PA in a cafe? It's worth thinking about.
Always try and locate your feedback before doing anything with the desk, usually you can do this by taking down a channel when feedback is sounding. If the feedback stops, you've found it's source. There may be more than one sound source (e.g. a mic and a guitar) which is feeding back.
There's quite a lot you can do with EQ to mitigate feedback, which is why big, expensive desks have big, complex equalizers. If you can find where the feedback is, try cutting a particular band to see if this stops the feedback. Usually feedback comes in at a single frequency, if you can cut that frequency, you might get more head-room without feedback. Remember, with EQ, a cut always sounds sweeter than a boost.
If you have a graphic equalizer on the master (So, not on a specific channel, going out to the mixer. You could try 'ringing out'. Set the stage up, but have no performers on it. Put every input up, then take up the master until you are close to feedback point. Put up one band on the graphic EQ, if it starts triggering the feedback, put that band lower than the others. Do not let the feedback get very loud or go on for long. Take the master level down if you need to.
You mentioned an electro-acoustic guitar (an acoustic guitar, plugged in), these are often the source of feedback. You might want to consider a 'feedback stopper' or 'soundhole plug' (they have many names) such as this one. http://www.gear4music.com/Guitar-and-Bass/Planet-Waves-Screeching-Halt-Soundhole-Plug/KJN
You also want to look into any effects you're using.
- If you're using compression, stop it. There's such a good chance you don't need it, but most compression units kick up the gain, feedback city!
- A small amount of phase, chorus or auto-wah might just clear it for you, particularly on the guitar. (or almost any frequency modulating effect). Obviously match this to the style you want to produce. Be very wary of setting up a stage with one of these effects on, then taking it off.
Sorry for the long post, but feedback really isn't just about positioning. Any sound tech will tell you they spend a lot of time fixing it, in a wide variety of ways.