How do I modify/add additional activites to my K-Bis?

My business is noted as commerce de detail d’habillement en magasin specialise, internet - APE K77IZ - Mode d’exploitation exploitation directe.
I am developing 3 separate activities in addition:

• selling objects/antiques/collectables as well as new home decor products that I purchase outright from a vendor at the professional marche aux puces/flea markets and possibly at open air markets in Paris (if I wish to broaden the home decor products base) this activity will be 1-2 days a week and seasonal.
• creating my own website where I will sell home decor products primarily
• selling the home decor products via pop up/concept stores whereby I will pay a monthly rent and a % of the sales to a company that runs the store, again probably seasonal unless it is successful.

My questions:

1. Can these 3 activities be added to my existing K-Bis? How should it be worded to encompass all of the above?
2. Can the modification be done online?
3. The CCI told me that I needed to mail my dossier to Nanterre and complete a form P2 but on the URSAFF site it appears that I can do it online. On the P2 form section 6 I don’t see much room to add info other than checking off the boxes magasin and marche. Is there something else that I am missing?
4. I realise that after my K-Bis is modified I must then apply for a carte ambulant to allow me to sell at the professional flea markets and open-air markets. Unfortunately, my first request was denied since my K-Bis was not modified first. I naively thought that the carte ambulant allowed me to do this automatically.

Any other roadblocks that I need to be aware of in hopefully creating my mini-empire?
Thanks as always for your help!