My panda 100hp will put the PAS light on if i haven't driven it for a month or two, but only ever the once on the first start, then it goes away never to return unless leave it parked for a while again. I've never worried about it myself, just one of them foibles of the car in my experience.
Further to which if there really is a problem with the torque sensor you do have issues with the PAS, we've had a mk2 punto that was totally fine but if you turned the steering all the way to full lock it would sometime put the PAS light on and the PAS wouldnt work until you turned the car off/on again, always the same error of torque sensor. I replaced the torque sensor and it never happened again. And on that train of thought, in the panda if i do leave it and the light comes on it usually goes straight out again if i turn the wheel all the way to full lock - Italian weirdness at its finest :shrug: although i've not had it come up in a long long time and the car does sit for a week or so quite regularly.
So without reading the whole thread back again i suspect there is very probably nothing wrong with yours if you don't use the car all that often - fiat and garages seem to tell you that replacing the torque sensor means a whole column but it doesn't - you can source a torque sensor on its own, no too difficult to change but do have to take the column off to do it and needs calibrating after (MultiEcuScan
can do it).
Battery wise, it should last at least 5-6 years in my experience, might be a parasitic draw somewhere perhaps. You can test it with a multimeter quite easily, put the meter in DC current, disconnect the earth from the battery and put one side of the meter on the earth lead and the other on the battery -ve terminal and read the current draw - there will be some but it should be negligible, if there is current you can then start pulling fuses one by one and seeing what circuit is drawing a current, a dirty and poor connection on a circuit thats permanently live with cause a higher current and drain the battery. Again that mk2 punto had this and it turned out to be the switch panel on the dash, it had clearly got water or something on it and i just wiped it all clean and fault disappeared.