I also have a '94 Auto with 108,000m, so let me see if I can help.
Definitely check that the new coolant sensor is the right type and you absolutely don't want to run the motor hot (anywhere near the red zone). These motors are notorious for blowing head gaskets and running hot will get you there in a hurry, by warping the alum. heads. The 2 rear cylinders have very little gasket clearance between the firing ring & the coolant passages, so regular coolant flushes and using distilled water are also a must to stop corrosion of that gasket.
Anyway, you should test all 3 fans on this car (2 speed puller cooling fan & 1 pusher fan in front of the rad). You can do this by running a self test with a code reader/scanner and check that the fans are running. I understand the extra front fan is a safety measure to help stop overheating if the main fans fail or can't cope. So doing that self test is a must.
In the meantime to be able to use your car I would suggest removing the CCRM (sometimes known as the IRCM) module which is bolted on the frame between the air cleaner housing and the headlight. Remove the module connector and locate the #14 & 17 wires (17 I recall is green w/purple or pink stripe). Cut the #14 wire about an inch or so from the module - with the ignition on that low speed fan should now run continuously. Put a female spade connector on the CCRM end of the wire and a male on the other. Install a 3M splice (which has a spade connector built-in) on the #17 wire and plug in the male end of your #14. What this does is run your low speed all the time & when the temp gets high enough for the high speed to kick in it will cut off the low speed fan. This is important because you don't want the high speed to kick in on it's own - it draws about 70 amps at start-up & you only have a 60amp fuse in the box under the hood. Older models use a fusible link which is much better for this condition. I found out the hard way and burnt-up that fuse (and fan motor), having then to splice in a new fusible link and new fan wiring (I'll tell you how I screwed up later). Now you have some safety built in to be able to use your car.
Also research this subject in the various forum articles, because there are some sophisticated alternatives using relays and adjustable fan thermostats as a final solution to the high running temperature problems with this car.
Originally I used a 200 deg sensor on my rad. tied into the high speed wire, so it brought my fan on early - which resulted in that fuse problem. Now I use the method above and connect the # 14 back together ( hence the need for the spade connectors) in the winter when you normally don't have these temp. problems & because it can put a stain on the battery at start-up in cold weather with the fan running upon ignition.
With regards to the mileage I get about 200m per tank doing short trips around town; but over 300 on the highway. So I'd start with replacing the O2 sensors, cleaning the throttle body (with safe spray cleaner), checking the PCV valve and finally cleaning the MAF sensor wires with alcohol using a q-tip.
When you push the overdrive button & the light comes on you have "disengaged the overdrive" or 4th gear. You won't notice much difference around town, but it will rev high on the highway. Try turning the switch off & on (around town only) and listen for the up shift.
As for the 'firm ride' the manual switch might be bad; but when you corner or brake hard and exceed about 84 mph that light should come on. You can also check each of the actuators on top of the shock mounts to make sure they haven't come loose and listen out for a quiet "shud" when you meet any of the activation criteria. That will tell you if they are working.
Hope that helps and definitely pursue all the diagnostic avenues for that fan problem, as blown HG's are very expensive. If bubbles in the coolant persist even when the temperature is normal you might just have blown that gasket and white vapor in the exhaust may not be very apparent with just one rear cylinder gasket failure.
Good luck,
Jim.