You are partially right, the K1 visa is kind of time consuming. Statistically, it will take 8-9 months for a Chinese fiancé(e) to receive a K1 visa in China. But on the other hand, we are strongly against the idea to use B2 visa to circumvent the K1 processing time. It is too risky. The reasoning is as the following:
Risk 1- B2 visa application
The immigration law requires consular officers to view every visa applicant as an intending immigrant until the applicant proves otherwise. Applicants must have strong evidences to prove that they have ties abroad that would compel them to leave the U.S. at the end of their temporary stay. It is very difficult for a divorced Chinese fiancé(e) to prove for that. In general, the passing rate of the visa interview in this category is not very high in China.
Risk 2- marriage and immigration petition with B2 visa in the US
Even though foreign nationals married to U.S. citizens are immediately eligible for permanent residence, the immigration officer may still have suspicions of your fiancé(e).
A. The immigration law provides your fiancé(e) with a legal channel to marry you and immigrate to the US by K1 visa, why did your fiancé(e) use B2 visa instead? It is very possible there were fraudulent presentation for his/her visiting purpose(e) during the B2 visa application.
B. B2 visa generally is only valid for 6 months in the US. If you are married to your fiancé(e) with the 6 months, then submit the immigration petition on behalf of your fiancé(e) consequently,the immigration officer may think that the marriage has been contracted solely for immigration benefits to circumvent the upcoming expiration of the fiance(e)'s B2 visa and to stay in the US permanently.

I am the permanent resident of the United States, can I submit the immigration petition for my brothers or sisters?

Unfortunately, only US citizen can do that.