Regarding the wrong NOC code: Yes, a wrong NOC code will lead to rejection.
Here is what I think you should do ....
If you have the AOR, you should have your file number; send CIC a letter clearly mentioning :
1) the new NOC code.
2) get a new reference letter from your employer... include everything from the CIC checklist. Because in the checklist they clearly mention what points should be covered in the Reference letter.
3) Include a detailed cover letter stating how you ERRONEOUSLY entered the wrong NOC code.
If you don't have the AOR (and a file number)... wait for it and do whats listed above.
Regarding NOA: (Who doesn't file their taxes? There is a red flag right there!

)
You can file last years and this years taxes all at the same time this year... make sure you do that. And include the NOA when you receive it. If you file it early you will probably receive your NOA by April. In the mean time, inform CIC that the NOA will arrive shortly as you didn't file your taxes last year. (I think it's better to come clean than to hide it. Because they can pull up ALL your data with your SIN... no point in hiding)
This is going to be blunt... but please bare with me.. because you missed these docs, you can expect your processing to be delayed. But I would suggest you inform them ASAP.
Hope this helps.