Bella finds out from J's "assistant" on the stoop that he deals in papers. This is why Alice sent Bella to J's office in a bad neighborhood, instead of his nicer office: she wanted Bella to find out what J does. Alice was giving Bella a way to make sure that Renesmee lived on. Since Alice's visions change every time someone changes their mind or makes a new decision, she couldn't be sure how the confrontation with the Volturi would turn out. So she wanted to give Bella a chance to get everything in order for Renesmee, in case an escape had to be made. Jasper has used J in the past to make papers, undoubtedly because the Cullens need to continue to make themselves younger and to give themselves new identities. So Alice knew that J would be able to help Bella make a new identity for Renesmee in order for her to escape the Volturi and have a chance to keep on living, even if the rest of them died.
Yes because J. Jinks is kind of sketchy and if she meets with his assistant she would be safer. Since Alice can see the future she wanted to make sure that Bella would have a back up plan for her daughter in case something happened.
Yes Bella did do the right thing by asking for the papers. Alice probably for saw that it was the best thing to do by sending her to the neighborhood to meet the young man rather than going directly to the main office where J. Jinks was. Especially since what J. Jinks specializes is illegal so it's best for Bella to keep low as well as J. Jinks under the wraps. Also Alice was giving Bella a way to keep Renesmee safe.