I have to say very old movie and I need to recall all things but here is what I found when I see the movie just for this:
As for your first question, there is only two man actually fall for Laxmi, Durgaprasad & Hari Lal. The main reason for this shown in the movie is both is widower since 20 years. Both of them have not needed anyone till they meet Laxmi. Movie covers too, when Durgaprasad first ask to Laxmi and Hari Lal tells same to Jai.
Now for your second question, what is there in her that they fall for her. For Hari Lal, from his first meeting, he accidentally (or later intentionally) dash against her and he feels that's special in last 20 years of widower life. He is also impressed by physical stamina when Laxmi takes stairs to run away from him. That she even follows tradition to bath before she cooks, or she bath while speaking mantras. He sees her as his dead wife has come to life again and he tells this to Jai as well.
For Durgaprasad, the scenario is pretty different. As Jai has taken makeup of his mother in law, he reminds Durgaprasad of his wife is obvious. But, from the first meeting when Laxmi was able to beat Durgaprasad with her wisdom (when Laxmi throws Bharti in water when she was on fire). Laxmi expose the thief servant and prove her loyalty to Durgaprasad. Also there are other good things in Laxmi like the fight she put up when the bad people harassing Janki or how she carries Durgaprasad when he has chest pain, make Durgaprasad fall for her. He also gets same believe as Hari lal that she is the one who can complete him.
The other people, Diraj is dancing with Laxmi, because he get to know the secret of Jai. Banwari Lal (Om Puri) is after Chachi because he felt jealousy for her. Joseph (Johnny Walker) knows Jai's secret from start, and he is there to just make money.