You will board with the preboarders as it is the only way that they can guarantee that you get two seats together. I generally let the wheelchairs get out of the way and board in front of the BS group. Sometimes the OA will see me and tell me to go ahead.

You do not have to have a blue sleeve. You can just show your extra seat card and preboard. As a COS, the middle will be empty and all three of you can not sit in the same row on one side.

The OA should allow the 3YO to board with you and may even allow the DH as well. However, they do not have to. Preboards that are disabled get to have one person to board with them to assist, and often times that turns into a bunch of help but it is supposed to be just one. Technically a COS does not need assistance and can be asked to board alone, but they have allowed my wife to board with me.

I would ask at the desk for a blue sleeve. Be prepared that they might not give you one as they have changed the procedure to allow COS with the Seat Saved document only to preboard.

The OA will see this and not question that you might need assistance, and your DH can board with you. Ask about the 3YO as well. You will all likely get to board together.

You and the 3YO will occupy a row with an empty middle and your DH can sit across the aisle.

It is not strictly by the rules but it may work. Just ask.

Finally, the DH and 3YO can board with family boarding after A60 and if you are towards the back of the plane it should present no problem.

One more question from an internet idiot. What is a DH? The only thing I can think of is "designated hitter" and that does not even work.