I am native AmE and mystified by others' mystification. There is absolutely nothing wrong with "They do not know him everywhere," and that sentence is not equivalent to "They do not know him anywhere."
-- "They do not know him EVERYwhere" means they know him in some places but not in all places.
-- "They do not know him ANYwhere" means there are no places at all where they know him.
actually, ho is the accusative of both on AND ono. ono is the other neuter pronoun available in czech, even if less frequently used than the demonstrative-ish to. there are contexts in which to just does not cut it and a form of ono is necessary.
Toto slovo lze v češtině použít i pro střední rod, ale mnoho lidí ___ používá jen pro rod mužský.
je, jej, and ho work fine here in the order of decreasing formality, while to would cross into the rather uneducated zone. (but then again, look who we just elected to second term.)
you may want to refer to the tips/notes provided with the Pronouns skill:
Several forms (e.g., ho, je/ně, and jej/něj) appear in more than one row of the table. Their meaning depends on context.