@ dharma2016
it is really difficult to suggest anything on this cases because from similar action different outcomes also coming about the amendments. So i think its depend on the specific application and the caseworker might consider others things too (might vary caseworker to caseworker too, they also might have internal standard to what to consider what not to) to get a decision that he/she should consider this amendment or not, shall they treat the amendment as a mistake or intentional.
We do not no details about your amendment figures before and after, also how much its affect your UKVI earnings claim so tough to say precisely but Sharing PERSONAL OPINIONS as I have gone through the same. Personally I believe the case worker handle hundreds of application so most of them must have very good idea about this issue. So it is really hard to escape hiding anything and better to explain everything if we think the matter is very much big for the application and could be decision making factor.
1. I think you must provide the amended SA302 or revised calculation whatever you have.
= I've explain this on my case in the cover letter because I think its better to explain before they think anything. Now, how you explain this is up to you and you can think how you can impress
and how you can justify that it was a genuine mistake (never write an essay but in short, in few lines). I'vent use the word amendment/amended but mention that there was a error on xxx tax year submission....... which I notice this and that time......... this and that way....... and then submit the corrected one after I notice.
2. yes without proving this there is high chance to get deception at this moment (unless caseworker ask for more doc, which they might not too).
3. None can say anything on this as same check now on both way. Personally I think 10Y option still you will have on your pocket if you go via t-1 but if you move to 10Y in the first place then you are limiting your option yourself (this is completely personal opinion).
4. You can take solicitor/expert advise but I think you should do which give you mental peace and looks more logical to you.
Good luck with your application, I hope you will see everything fine at the end.