Only get 1 chance? What's the success rate on asking for a K1 a second time?

[American 1911]

Basically, God forbid, if she goes home during a K1 prematurely (eg: we have a disagreement about the wedding or there is a family emergency), what are the chances a petition for her using the K1 a second time will be accepted?

How big of a No-No is this? For example, if she leaves the US after 2 weeks, because we never got married? Does USCIS believe in second chances for the same person if they didn't overstay or abuse the visa?

Do I need a reasonable excuse?

How often does this happen?

Me: US citizen, 28, never married, 30k+ salary
Her: Russian, 28, never married, low income/no assets, B2 (sponsored by me) declined summer 2017.

3 years contact, 2 visits, both by me only in 2017 so far

[inadmissible]

You will have to get a waiver if you want to petition for her (or any other person) again within 2 years

K-1 is a fiancee visa, it is not a girlfriend visa. Use this process when both of you are serious and are actually committed to getting married