can
Just not fast charging. The Honor 9X does not support fast charging, and supports up to 10W charging.
Can be used, but will be limited to 10W, after all, 9X does not support fast charge.
Of course you can. Nothing to worry about. Even if the original charger is not used, it will use the safest way to charge. Almost all mobile phones support safe charging methods.
Honor 9x does not support fast charge, Honor 9X is positioned in the low-end market, with Kirin 960 processor.