If you aren't willing to read the evidence, then I'll post it short and sweet.
-Goku wants to transform using potara.
-Old Kaioshin says he can do it after merging.
-Goku does not ask any questions.
So my question to you, p123, is why does Goku not ask any questions? Is Old Kaioshin apparently knowledgeable on how Super Saiyan merges work? I would think not. Goku should be saying "We can't transform when using the Metamoran fusion type, are you sure it'll work with potara?"
And even so, who is Old Kaioshin to say that it will work for sure? Seems to me that Goku can already do it with the Metamoran style.