You have posted my million dollar question.
"Sinoegee of the family Spanalgee" written in Hawkin's journal dated Dec. 12 and 14, 1796.
I have research and found nothing. I cannot find any words close in the Muskogee language for Sinoegee, one problem may lie in the translation from Muskogee to English. Even though the Muskogee language uses a alphabet, the sounds of letters are pronounce very different than English, example the letter 'r' is pronounce 'thle'.
May we give thanks to the 19th century autotheist of the Muskogee language for that brilliant thinking. NOT!
Now, Spanalgee sounds both Spanish and Muskogee, who knows, I cannot find this name anywhere esle. I am not sure if its a clan or a family name. Remember the Muskogee language has many different dialects and tongues. Many of these no longer exist today. Maybe its part of a lost dialect of the Muskogee language.
The name Grayson is spelled KELESEN in the Muskogee language.
I am not 100 percent sure, but my dollar will be on Sinoegee's family (if she had any) taking the name Grayson as well. If she had brothers they might have taken their wife's family name as was custom in that day. One thing for sure there are a lot of Full-bloods with the name Grayson and they sure did not get it by descending from Robert the Scott.
The people you mention in your post I have no ideal who they were, do you?