Genius, it makes good sense.
Specially since its just 1 generation or whatever many generation before Madara vs the first
i doubt this theory tho. Just the magnitude that if the first sage of six didn't last forever why would his son
also i think the sage of six paths didnt just split off the tailed beast, but also split off his eye powers and body powers, well obviously
first to his oldest son which was turned into 3 different eye technicques, a weaker version of the rinnegan, sharingan and byakugan
then splitting his body powers between the Senju, uzumaki's and _____

idk
I just don't think it can be Izuna, it leaves a bad taste in my mouth thinking its Izuna.. same with shisui but both are very viable choices and can't be disregarded.
Not only that if those 2 are possible might as well throw in Obito.
The fact that he has Madara's chakra is very interesting.. although he is Not madara himself he can control his chakra and has a portion of it thats powerful.
Now that being said, he may of recognized the Chakra, but Tobi may just have very similar chakra and thought it was madara by mistake
which leaves direct relationships with Madara the best choices.. Izuna, his Father (possibly older son of the Sage) Madara's son, Izuna's son if they had any
i really wanna assume its someone i just dont have anymore choices. so im just gunna say its none of them

This is true, if it was shisui that did it he had STACKS ON STACKS of eyes. he didnt need it and he needed an alibi
This is another factor that I took into consideration when thinking about it being the Sage's older son. If that were true, then he would be one boss ninja. Why would he be scared of someone like Madara being brought into play, unless he was just afraid that his jig would be up?
I too find it interesting that he had Madara's chakra. That leads one to think that it could be Zetsu related. Or, it could be someone in direct relation to Madara, perhaps his son... or Izuna.