russcore

I second getting married somewhere that it is legal and doing DCF. This is by far the fastest, cheapest, and least-problematic (can work & travel right away, no AOS in the US, etc.) option.
Other than that, do what you need to do.
 
Tax returns would be for the Affidavit of Support (I-134 for K-1, I-864 for CR-1), which is not submitted with the petition.
For a K-1, this comes into play at the interview.
For a CR-1, this comes into play at NVC.

Can't specifically answer some of your questions, but you can take your questions and search for them in the search bar on top right of the screen and you will find countless threads with information.
 
While K-1 seems "annoying"... as mentioned, for some of us, it is really only way to go so. Key thing, don't over think anything.

US citizens are required to file taxes regardless if their income is from China. You will need to submit these with your petition.
 
Your mother isn’t the sponsor- you are. Your mother would be the joint sponsor.
 
There’s no requirement for taking trips together lol. You need to show you are both free to marry and have met within the past 2 years. I’d imagine you have hundreds of photos together that you can pick and choose what to submit and state the place they were taken at.
 
You will have to establish a place of residence in the US. Will your mother let you two live with her? I also imagine you would start looking for a job to establish yourself in the us as well?
 
In describing your meeting -we submitted a one page typed paper outlining how we met -dates, why I was in Haiti, how I eventually met my husband, how we stayed in touch etc.
 
All forms should be filled out in English - you can read them to see if you should be writing both his native alphabet with English or just English. I’m sure it states on he form though. 
 
If if you are able to translate Chinese you can translate it yourself with a note stating you are competent to translate from (language) to English and you swear the translation is accurate.