I own an earlier (sn 150, early 90s) JP80. In answer to your questions:
1. While I have often been tempted to try another pre, and due to a chronic problem with the phono stage of my JP80 I have actually recently bought a Lamm phono stage, there still is no preamp I'd rather own, and I've had mine for about 10 years. Adding NOS Telefunken 803S tubes to it just reinforced that feeling!
2. I have never heard of a JP80 with a solid state power supply. Are you sure it's not tubed? There are only two tubes, an EL86 and an EF 86, in the power supply, towards the back, maybe you didn't notice them because of the size of the transformer?
3. Jadis didn't start using balanced inputs/outputs until recently. No early 90's one should have balanced connections.
4. I've similarly never heard of a JP80 without both a MM and MC input. You should check the tube complement--if there are two 6DJ8s towards the back and parallel to the back, and then 3 12AX7s to the right of them heading perpendicular to the front, you should have an MC stage with about 60 db of gain. No 6DJ8s, then there is only a MM stage. Notwithstanding the problems (RFI induced, I believe) I've had, I'd say the Jadis MC stage is superb, though a little noisy in comparison to today's stages (the Lamm is infinitely quieter, dead silent). Part of this is due to the use of 6DJ8s, which tend to go noisy quickly and are run at a very high level in this circuit, making them prone to wearing out quickly.
5. Can't speak as to counterfeits, but I would suggest that you get the serial number and run it by Jadis by contacting them through their website at Ecrans Jadis, as they note the problem themselves at the website. My unit looks like the ones you see today from the front, but the tube and cap layout behind the faceplate is different--the caps are black, with Jadis written on them, not the blue caps they started using a few years later, the caps are not lined up in banks as on the new ones, but rather are in various places around the tubes, and the line stage tubes (in addition to the MC and MM tubes I mentioned above) are behind and parallel to the front plate. Also, the twin volume controls are not linked, as they started doing about 1993 or 4. My face plate has, from left to right--mute switch, tape loop switch, MC/MM switch (that could tell you if you have an MC stage), source knob and the two volume knobs.
6. Unfortunately, I don't have any pictures that I can put on the net; hopefully my description of the front plate and the tube layout is of help.
You may be looking at one made for a non-US/European market, but it does not sound exactly like my early-90s model. I would contact Jadis in France, they should be able to help and are concerned about counterfeits. If this is the real deal, while there is some unit-to-unit fluctuation in sound, on the whole the JP80 is one of the most natural and musically satisfying preamps on the market, although it, in its original version, is not the last word in transparency and can sound slightly romantic, a little rolled in the highs and the bass. It is better suited for classical and jazz, as its dynamics are voiced to sound like what you'd hear in a concert hall, rather than a close up, instant-on/off dynamic you'd get from rock. Hope this helps to some extent. If I can find a picture to post on the web I'll let you know.