TWISTIE

You're welcome, and as others have already said, your current annual income is the key factor. That doesn't mean a CO can't deny if you are currently earning enough, but that because you are currently earning enough to support the beneficiary, you could be ok on your own without the need for a joint sponsor.

Correct, and to clarify.  If you had 50,000 in income for 2020 and lost your job last week, your current income is zero and you do not qualify.  On the other hand, if you earned nothing last week and started a new $50,000 a year job last month, your current income is $50,000 a year and you may well qualify on your own.  

Yes

First, let's clarify terms.  For an I-134, we speak of a "co-sponsor".  Allowed pretty much everywhere but Thailand and Philippines for a K1 visa.
 
For an I-864, the correct term is "Joint Sponsor".
 
There is no prohibition against using foreign assets but still a judgment call based on totality of circumstances.  To understand use of assets better, study that section of the I-864 instructions.  Pretty much the same judgment principles apply when using the I-134, BECAUSE in a K-1 case it is expected the I-864 will be used later anyway after marriage.  (When adjusting status after marriage)