Her arms don't have to be covered, Zara is proof of that. Simply her shoulders.
Why? How is she comparable to Anne or Camilla, other then second wedding?
Anne and Camilla were older women, both with semi grown or grown kids. Often when married older in life, even if first marriage, you avoid the more formal gowns and veils.
Anne had a church wedding, but it was a very small affair. Camilla had a civil ceremony. A veil and full gown at a registry office??? Even if she was younger, or first wedding, or both, it would have been over kill.
Both women had the huge church wedding. They both had the huge gowns and veils and trains the first time around. The reason many second time brides forego this kind of thing, is because they have already done it.
Meghan married on a beach and had none of this. She is not being married in a small private ceremony like Anne. She isn't being married in a registry office like Camilla. She is being married in a grand chapel (would be considered a cathedral if not part of the estate). All of this will obviously be taken into account.