Alternator wire was a blind shot in the dark. Doubtful that's your problem. I'll assume it isn't.
Some people claim mpg gains after cvt fluid change. I try to replace it on the fleet cars every 30,000miles(ish).
I don't pay much attention to the average(trip b) lifetime fuel economy on the work cars. For my personal 2014's (both cvt)that I have owned, they were pretty close to the same. One car had about 30k miles(then engine was replaced under warranty for knocking) then I sold it last Feb when I bought my fancier one with 283k(176k miles) with a questionable cvt. It got similar fuel economy with that many miles as my lower mile car did. I replaced the engine(and cvt) with ones that had under 6000miles after it developed a bad rod knock. Economy went up slightly for a little while but I blame that on me trying harder and running the stock dunlops. I've been running taller 175/winter tires since last fall(ish) and manage around 40-43 mpg(mostly driving at 50-55mph). 28 seems awfully low.
I'd consider a cvt fluid change and maybe replace(and inspect) both filters. Or at least replace the cartridge filter and drop the oil pan to get the most fluid out and inspect the pan for treasures.
I just use universal cvt fluid that meets the J4 spec. Seems to work for me. These things are pretty picky about fluid level. FSM says to use a scan tool to measure fluid temp then compare to chart. I usually just measure what comes out and pour the same amount back in. When I do just a drop and not change any filters I usually get 2.5L(2.6qt) from the drain plug with the car level on a 4post hoist. That might be able to get you a rough baseline if you figure your level is incorrect?
Mirage videos:
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View my fuel log 2014 Mirage SE wussie cvt edition. 1.2 automatic: 37.7 mpg (US) ... 16.0 km/L ... 6.2 L/100 km ... 45.3 mpg (Imp)