I have seen mistakes on wedding docs for father's name. It drove me crazy , I had one that said it was Samuel , so everyone had Samuel in their trees. I took the time , did the research and it was definitely John. A clerical mistake...
It said Thomas on William's death cert ...
There is for sure a Thomas Grimason who is a weaver near Portadown at the right time. But there is no candidate for an Archibald Grimison weaver anywhere.
Other then Sarah and Elizabeth whom I guessed were his siblings , I have not seen any list of brothers or sisters for William Grimason 1836-1918. Have you seen any reference to siblings for William?