Reporting a profile reports for information on the Bungie.net profile on the site, and that report is sent to a different team than the ones that deal with in-game reports. Reporting the profile itself should not be done for in-game activity.

Unless the profile itself has some issue, such as their name being inappropriate, reporting a profile doesn't do anything, and, on the contrary, reporting a profile multiple times, or reporting multiple profiles that do not have any such issues may lead to restrictions for inappropriate use of the reporting tool for false reports. Manual reports for in-game activity, messages, and the like should be done from the game history, or using the manual contact form I mentioned to the OP.

Submit the information you have. If you have a picture, make sure to submit it as well. You don't have to have the specific Bungie ID with #, as Bungie has their own ways to see who you were connected with and identify them. Give as much information as you can in the extra comments section when submitting the form. That's all you can really do, there's no other method of reporting if you cannot report them from your game history.

Yes. While the form name may be misleading as it hasn't been updated to reflect the full use of that form, since it is for reporting things other than cheating, the page name is a better description: "Report Suspected Cheating and Poor Behavior". That form is what you need to use to report the behavior you described in your initial post.