K1Applicant1

I don’t think the IRS will elaborate on it until they come out with the 2020 tax forms.  My example is based on the 2008 stimulus which was handed out early but when the 2008 tax filing started, the forms and instructions were adjusted to have additional lines which covered eligibility for the 2008 stimulus. My wife had received her stimulus before we had even filed the I-129F. By tax filing time, we were married and filed a joint 2008 return. It was much more long-winded than my example as the IRS always is, but the results were we qualified, then her early payment was deducted as received and I got my share added to the refund. 
 
The federal government created the 2020 stimulus and wanted to get the money out early to help those in need, so based it on previous returns rather waiting for 2020 tax filing season starting next January. They call it the 2020 stimulus, not the 2019 stimulus They refer to the checks/deposits as “early” payments.  
 
Your spouse would have had to be on a  2019 Joint return with a SSN to get the “early” money. That does not mean she can’t qualify for the 2020 stimulus from what I have read. I feel sure it will be added to the 2020 tax forms based on the last government stimulus.

Assets are optional. As long as you don't need assets to prove the sufficiency of income then you do not need to fill it. This does not mean the form is not complete.

I left blank,  

It's not 100% clear if scanned, signed, and emailed I-134s are okay, though many people have had no problems with them. Check with recent applicants through your embassy/consulate.
 
It's certainly okay send the supporting documentation electronically; the I-134 is the only thing your fiancée needs from you that has to be signed.
 
It's probably best to get a new statement; it will have been several months between your initial statement and filing for AOS even in the best case (going by VJ averages, it's 53 days from NOA2 to embassy/consulate, on average 42 days to interview in Japan, and then your fiancée needs to get her passport back with her visa in it, get ready to leave and leave, then you need to actually get married and even if you do that shortly after she arrives it can take a few weeks to get your marriage certificate before you can apply for AOS; at any rate, it was about 5 months from I-129F NOA2 to I-485 NOA1 for us). So a lot could have happened in that time.