I have seen your post about this topic also in "Alfistas" where I appear also as "Garbancillo" (by the way, I am also based at Madrid).
First, check that battery is Ok. 159´s tend to spot some warning lights and messages just before the battery dies. If you are running out of battery, it may be possible that the fuel gauge issue is related also to a low charge level of the battery. However (at least, in my case) such odd messages and warning lights come out of the blue just before the battery runs out completely (I mean, it would be rather strange that such messages and warnings are there for a while, let's say for some days. What I have perceived on the matter is that battery will die inmediately thereafter, in no more than a few hours.
In my particular case, the only time that battery came to an end, the fuel gauge reached only the very first quarter of the scale, becoming then affixed in such position instead going back to "0" when you put the Start/ stop button in off mode. When you replace battery, all is normal again and all warnings and messages dissapear.
Then, if the battery is OK:
1. Double check the clutch sensor. I would recommend you to pull the clutch pedal and go then for a ride to test if the "VDC/Hill Holder unavailable" appears again on the screen. Anyway, replacement of the sensor is easy, cheap and will take only a few minutes.
2. The issue of the fuel gauge is not an iddle one, may be the float/ leveler into the fuel tank is not working properly (I would disregard problems with the fuel pump).
Anyway, I read also at "Alfistas" (for all of you, the Spanish forum of "Alfaholics") that you acquired the car recently
1. In Spain, you know, if you acquired the car form a private vendor, this shall bound by operation of law to repair these tricks whether the appearing problems may be regarded as a "hidden defect" (In England, "fitness for purpose", although our legal institution of "hidden deffects" is more protective) We call it "saneamiento de la compraventa por vicios ocultos", Article 1484 of the Civil Code. You are entiltled to get these repaired by seller during six months from the purchase date.
2. If you acquired a used car from a professional vendor, the above is not applicable, but the provisions set forth in the Royal Decree 1/2007 ("Real Decreto legilsativo 1/2007, de 16 de noviembre, Texto refundido de la Ley General para la Defensa de Consumidores y Usuarios"): You are covered by "warranty" which in no event may be less than one year from the purchase date, and such warranty is absolute, unlimited and unconditional.
So, in any of 1 or 2 above. go back to the seller and seek to do the needful on your 159.