If it's not enough, it's not enough, regardless of how much flexibility it offers.
living in the past, you would not know of hardware IIC, hardware SPI for both expanders are available

Well, when they tell you that if you don't use this programming language, they won't support you, then they're off my list.
I do not know who you talked to, I have had no problems with support for my 1-15% assembler stuff.

I'd think you'd find that quite self-explanatory. While the datasheet provides plenty of information, it says nothing about the timing of the 'C' code. The 'C' code tells you nothing about the timing, too.
obviously, so HOW IN HADES DO YOU EXPECT
how long something takes. When it's in 'C' ... well ... they can't do that, can they?

the support folks would ordinarily be able to tell you how long something takes.
where did you find such a wonderful support, I never did (at ANY supplier)

Erik