I know I've already weighed in on this, but I'd also like to point out that in the Druid anathema section, it states "Each druidic order also has additional anathema acts, detailed in the order’s entry."
My reading of this is that each order has all the listed anathema for Druids and additional ones, because otherwise it should say that each order has it's own anathema acts rather than say they have additional ones. "Additional" anathema would require the order to already have anathema to add to.
I understand that some people may interpret this a different way, but by the rules I've laid out so far it would take quite a bit of stretching of the rules to enable you to learn these abilities without becoming a Druid. A GM could allow it to happen, but I would not consider it RAW.
I also feel strongly that if you multiclass, you are becoming a member of both classes. You just don't have the abilities of someone that trained in that class for a longer amount of time as their main focus.
One thing I don't understand is why this is so limiting to people. There are other materials to make weapons and armor out of and if you are going to modify rules, it seems much easier to reskin existing armor than to erase core class concepts.
I will say that as a general rule I dont read "additional" as going both ways.
If you order a steak at a restaurant and it comes with an additional side of potatoes, do you assume that a potato comes with a steak?
Edit* To clarify, the archetype doesnt refer to the order anathema as additional, only saying that you must abide by that orders anathema. This is a case where I believe they are saying order the potatoes. If the archetype referred to the order anathema as additional, I would agree with you.