It should be straight forward like that, but you might need a scan tool to remove the airbag fault code.
Make sure the connections are actually solid by using proper automotive crimps in an appropriate size and test the connection by pulling on the wires once crimped. Soldering is not...
You should be able to just clean the sock well enough. The tank is plastic so it should be able to be rinsed with water, the only major rust source is the filler neck.
Have you checked the fuel pump and its filter sock inside the fuel tank? The filler neck tends to rust, which then get sucked into the filter sock clogging it.
That will cause low fuel pressure, which the car can attempt to compensate for by firing the injectors for a longer period...
I would check:
idle air control valve
fuel pump
ECU coolant temperature sensor
I had similar issues with mine, unfortunately I got rid of it before I could change the fuel pump.
Where do you keep your fob when attempting to start? Maybe the signal is getting blocked by something like a RF blocking wallet?
Check your owners manual, they are usually pretty thorough about things when it comes to key placement and interference.
I wouldn't worry too much about it then if everything else in the car works. If it's causing trouble best to remove the whole system and install a modern one if needed.
Also, that lock seems quite easy to pick if needed.
From what I can recall, @yoeddynz had a similar problem, described here: https://micra.org.uk/threads/kiwi-k11s-car-number-five-scuttle-shake.72948/post-794664
Looks very much like an alarm siren. I'd bet it's a dealer installed option, and likely not many were fitted so there's no information on the internet.
Removing a front bumper should not be too difficult and should not need you to jack up the car in order to do it. You might need to turn the wheels for access to screws/clips in the front wheel wells and some crawling to get to the bottom, but that all should be doable in any parking spot.
Check the fuel tank for stuff at the bottom clogging up the pickup.
Also, another thing that might be failing is the ignition control module within the distributor. Google RSB-53 for the part.