Fallout – How to build power at the home in Diamond City

No. The "Scrapper" perk applies to weapons and armor, which can not be auto-scrapped in this way.

Although you can mix and match quest lines and switch faction affiliations at various points in the game, you have to pick one faction to help you build and operate the molecular relay.

This will be one of the Railroad, the Brotherhood of Steel, or the Minutemen. Having the Minutemen aid you could easily be referred to as "doing it yourself" since at that point you're the leader of the Minutemen, and they would be acting on your orders.

To be clear, you can have only one faction help you build the relay, but can still switch which faction you ultimately support later. If you've already completed the faction quests with each other faction up to the point where you could have asked them to help you build the interceptor, then you should be able to continue those questlines with them regardless of who actually helps you build it. If you haven't been doing quests with the faction up to this point, then you might get locked out.

For more details on this quest and the steps needed for completion with each faction, refer to the The Molecular Level page on Nukapedia. Official guides often contain errors, but errors on the wiki are usually fixed, due to the fact that it's a wiki.