No, you cannot meaningfully delete your posts or comments, but that’s not because of any issue with lemmy, but because you posted them publically. They will be archived and indexed in other services.
It is always best to remember that all your activity here is public, and will be linked to your username. Given that, you may wish to minimise any personally identifying information you post, and use several accounts to split up your activities by topic.
My kids have a pile of cardboard screws that they use to turn boxes into all sorts of things; rockets, forts, cars and you could probably make organizers, shelves and the like too. The screws grip the cardboard surprisingly well, and it’s easy to make even quite large structures robust.