Response to your questions
1 - I am afraid, we may regret not getting married for cr1 visa because i heard fiance(k1) visa is taking 2 years, is this right?
K-1 visa for us (I understand everyone goes through different lengths and time for various reasons) has been 2 years to get to the second step of process. I've known others in similar circumstances who were ahead of us because they were on the marriage visa route. We still have at least another year, so 3 years total in our k-1 journey.
2 - I am afraid, uscis doesn't care about k1 visa and may ignore it or let it wait, is this true?
We had some complications with USCIS and NVC tossing our application back and forth - I am not sure why but cr1 visa route may have avoided this. I don't believe they actively ignored any visa, but others may be prioritised.
3 - do most people regret choosing k1, i heard mixed reviews. what would you do if you were in our sitatuion(please review our option 1 and option 2)
I can't fully say I regret anything yet until we've made it through the process. Because of other factors (work, life) we are not in any rush to get to the states, so if time was of an essence, the cr1 visa would have been more advantageous.
4- is it ok if we have a cultural/religious ceremony(not official marriage) after we applied k1 visa? We will only be engaged but have a small ceremony.
Not sure.
5- what would you do if you were in our situation? wait 3 months and apply for cr1 or start k1 right away(our goal is to be together in the usa, as soon as possible)
K-1 visa will take longer it seems like, so applying for cr1 "might" be the best option if you want to decrease the wait, but again haven't not gotten through our own process - I cannot say for sure. I think everyone's situation is incredibly unique and the process far from easy. Wish you and your partner the best.