Erratic idle control is just one of the many K12 issues & it looks like you have worked through most of them where most people give up & get out PDQ? A few simple tests & checks spring to mind in the following order, before spending any more hard earned money on it.
- Clean up the engine bay earths as these get corroded on k12s and can cause numerous faults.
Two under the o/s headlight, 1 is on the top metal bar above the bonnet latch.
Two under the n/s headlight and 1 on the bulkhead near the abs module depending on how old the car is as Nissan moved the earth point as it caused abs faults. - Test eng coolant temperature sensor resistance correlates to Haynes Manual specs?
- Disconnect crank sensor to test if it idles better disconnected?
- Crank sensor connector swells up and makes a bad connection on the earth - to fix this you have to solder the wires directly to the sensor also you might have to change the sensor again.
- Test continuity of all sensor connector wires back to the ECU.
- Ignition switch faulty is common on k12 - Nissan put out a recall years ago, thus to check if ignition switch needs to be changed?
Remove the ignition switch and see if the T shape bit in the middle is black or white,
If black then you need a new modified one as this might fix the fault,
if white then you already have a new modified one fitted. - Next is to continuity test ignition switch all functions, looking for intermittent operation.
- Chain tensioner piston is out by 10mm or more the chain is stretched.
References:
https://www.honestjohn.co.uk/carbycar/nissan/micra-k12-2003/good/
https://www.honestjohn.co.uk/forum/post/index.htm?t=33935
Good will hunting 
