Immobilser Issue? 2015 Fiesta SE not starting

I have a 2015 Ford Fiesta SE with 38,000 miles and manual transmission located in Las Vegas.

After sitting for 3 days it wouldn't start or have ANY electrical power for the dash, radio, lights etc. The battery had 4.58v and 10.25A, which explains the complete lack of power and the negative battery terminal had blue corrosion (normally indicates undercharging). I tried to jumpstart it but no luck although all of the dash lights, radio etc started working again when the jump leads were attached.

I bought a brand new battery which currently has 12.4v in it after being in the car for 2 days. When I try to turn the car over the fuel pump turns on and I get the initial click from the engine area but nothing else, the engine doesn't even attempt to turn over. I have no warning lights on the dash or error messages either.

I can see two options as to why the battery was drained, the alternator wasn't charging correctly or something has drawn power from the battery over the 3 days it sat there. However, I cant leave the lights or the radio on as they switch off once the car it switched off and the internal light was off. I was thinking the immobiliser needs a reset but according to several resources online this has to be done by Ford, which makes sense otherwise thieves could easily bypass it.

I dont want to take the car to Ford yet as it will cost me a small fortune for them just to look at it. Before I completely run out of options, are there any quick fixes or ideas on where to start to diagnose the fault?

It is used as a commuter car for the daily 45 mile run to work and then back.

Any help is really appreciated as I can't afford $1,000 for Ford to change every component in the engine one-by-one..... :-(