It is not easy that much is not a sentence that would ever be used, although I think grammatically it is actually OK, as it follows the same structure as say:
It is not easy at all - fine
It is not easy in the slightest - fine
It is not easy that much - sounds weird. As you said, "It is not that easy" would be said instead.
I wonder if that one is also able to be used
Again, grammatically there is no issue, but this sentence sounds a bit "clunky"(for want of a better word). Simply replacing "is also able" with "can" makes it much better:
"I wonder if that one can also be used"