You do not
need to uninstall TeXLive. ConTeXt Standalone can work side by side and it's
very simple to install. You can pick to use the standalone for some of your compilations.
\persiandecimals and \arabicdecimals are very similar. The difference is (almost) only in digits for 0,4,5,6. However, Ibn Saeed most probably needs \arabicdecimals to typeset "mashiriqi" numerals.
The important thing is that neither one exists in TeXLive Context. It was there in MkII but not in MkIV for some reason. It was added to MkIV sometime in the fall.