Your choice 1 is all I had to do as far as getting my SB6121 activated. I had no problems getting it activated just by using the walled garden, but mine was a new SB6121, not one that had previously been assigned to a residential class service.
You say that you switched over from residential to business class, but I don't think that is possible since residential is handled by Xfinity, and business class is still Comcast (two different entities). You will probably have to wait until your old residential account is completely closed (and that can take weeks) before you can get Comcast to accept your old SB6121 for use with your business class account. Several years ago when I had a residential account and moved, it took several weeks (and the intervention of the local franchise authority) before I was allowed to activate my D-Link modem at my new address because Comcast said that it was already being used by "another customer" (me at my old address).
Even after you are allowed to use your SB6121, you may not be able to simply return the SMC gateway to your local Comcast office. My local Comcast office told me (after I waited in line for an hour and a half) that they did not accept returns for business class customers, and I had to arrange for a service call and have a tech pickup my SMC gateway.
As for contacting your sales rep, good luck with that. My sales rep has never returned a phone call or email since the day I signed my contract.--
We can never have enough of nature.
We need to witness our own limits transgressed, and some life pasturing freely where we never wander.