As John says... Get a thorough code check done
However, considering the many faults you are having all at once, it suggests something far less complicated than an ECU replacement......
Why do people automatically assume it's an ECU problem is beyond me???? If it worked before, then there is no reason why it has failed no short of being hit by an alien EMP weapon or struck by lightning
What I am saying... Is, it is a very rare occasion indeed than an ECU goes wrong!
Free things that you can check first.
1. Is the positive clamp attached to the battery properly?
There are two rings top and bottom on the clamp, if the top ring isn't seated on the battery terminal itself, you WILL get intermittent readings and loss of signal due to vibration and engine mount natural movement. A simple loosening off, tap the ring down with a screw driver and nipping up will suffice. While you have the plastic fuse type cover open, remove all the bits and cables, clean up the contact and fit it back together.
2. Is the battery healthy? I know you have had the alternator done but was the battery changed to?
Any tyre and exhaust centre can do a FREE CHECK on the battery using what is called a "drop test"... They simply arc the posts for a short time with a specialist meter that tests the condition of the cells and measures the capacity of how long the battery can resist the state of discharge in comparison... A battery can look healthy with 12.5v+ but if it drops quickly then you aren't going to get much voltage or amps to actually help the ECU read the signals properly.
3. The most common issue.... Earthing! The ECU has its own earth, a small wire attached to the outer case, follow it and make sure the wire is healthy and has good, made contact points, remove them, clean up the contact points and bolts with wire wool and also the outer edges of where it screws into and refit... Don't automatically assume it looks ok, so therefore it is!... Its a free fix that takes minutes to do!
Also do the same procedure to the negative terminal connector for the battery and cable to the inner wing.
I would be performing the above inspections and fixes and checking if it cures the issue after each one has been completed.
Still no joy?
4. Ok... Now we move onto the ECU area... The two spring clipped multi-plugs... Have these been off recently?... Disconnect the battery first....Un-spring one clip and lift up the locking mechanism fully. Carefully lift out the multi-plug.... Inspect the pins and contacts carefully, any bent, missing or out of shape?... It can happen with careless removal and refitting! Can be rectified by gentle movement of the offending pin back into shape if one is bent. Finish off by spraying electrical contact cleaner onto the male and female blocks (available for £4.00ish from Maplins)... Allow 15 minutes for the excess spray to dissipate and refitt it gently back into the socket and push the clip into the secure position... Now perform the same task to the next ECU multi-plug...
Reason why I say one at a time, is you won't believe the amount of ham fisted muppets who actually try and force each plug on the the wrong pins even though they are slightly different and end up wrecking the loom and ECU in the process
5. Other simple checks that can make a difference and can often seem completely unrelated at the time but since you are rooting around, you might as well do...
In front of the battery are relays under the cover.... Remove them one at a time
while you have the battery disconnected anyway... Clean up the contact points on the connector blocks and the spades of the relays themselves... Electrical contact cleaner helps again... Refit and move to the next one just to save any confusion of which one goes where.
6. Now some simple logical fault checking... So you had the alternator replaced?
Now is the time to check the cables to and from the alternator just to make sure it is doing it's job correctly, clean up and refit them... Follow onto the starter motor contacts ( you might as well clean them and refit as a matter of maintenance anyway while you are there and lastly
7. Pay attention to the gearbox mounted earth cable.... This is more important than owners realize and a lot of the loom takes it's earthing point from this humble earth cable and bolt... Replace if it's groggy and worse for wear, they cost a couple of quid from any motor factor.
8. Make sure the battery is firmly connected again (assuming you have had it drop tested and it's passed the test) and don't forget to double check everything for security of fitting... Follow up by checking the fuel cut of switch under the front passenger seat hasn't tripped, it can happen when tinkering with the electrics.
9. Get under the glove box area and locate the throttle pedal sensor multi-plug. Disconnect it, clean up (electrical contact cleaner is now really payng for itself isn't it?)
Refit it together firmly and run a thin cable tie between the wires on both sides of the plug and secure it tightly together.
10. Ignition on and check that the code check light for the immobilizer goes out after a few seconds
11. Can you hear the fuel pump charge initially?
12.If that's ok, now the simplest of checks that anyone can forget, how much fuel is in the tank itself? If it's low on fuel and you haven't been able to start her for a while I would always advise putting a further ten litres of fuel in, just in case... 147's,156's and GT's all suffer the same issue when low on fuel due to the fuel pump pick up location and tank design... They can be a pig to start if lower than ten litres of fuel is in... What have you got to loose? once the car is running, hopefully... You will be using the extra fuel anyway!
So there you have it... Free and cheap, easy checks first, you never know, it might be just that simple
Then you can blow a wad on an ECU if you choose but at least you have got the basics right before wasting a lot of money first if it isn't the ECU...
Oh.... and find a better Auto electrician who has worked on Alfa's before.... A good one would have checked all the above as a matter of course before scratching their head