This might help us sort this out - Wiki's l
ist of guests at William and Kate's wedding, including those who declined. Notable:
The US was represented by the US Ambassador, and it appears that no other US officials of any kind were on the list. Now of course this could have been arranged in advance, but given the fussiness over protocol I would think if the Ambassador was attending in place of the President, that would have been noted under those who declined.
For example, the Prime Minister of Canada was invited (Commonwealth countries), but declined and was represented by the Governor General of Canada, who was invited in their own right (again, Commonwealth countries).
As a person not holding any current office, I would think the Obamas fall under the "friends/celebrities" category and we'll see the US Ambassador invited as the official representative of the US, exactly as it was for William and Kate's (more important anyway) wedding.
I get what you are saying that it's not about protocol but rather Trump's ego, but since when did the Royal Family abandon protocol and tradition due to an American's whim? IMO the family is firmly circled around Harry and his bride and will stand by him, with protocol, precedent and tradition as their armor.