Even if Apple invests in Toshiba's unit there's no way they're going to use them as a single source. They might give them a guaranteed percent of their NAND orders (especially if they get favorable pricing). It would still be in Apple's interest to shop around.

Also they already design memory controllers. They purchased the Israeli company Anobit in 2012 to do just that [1]. That's why Apple's storage benchmarks on recent products are really good.

[1] https://en.wikipedia.org/wiki/Anobit