Why can you use "içer" instead of "içerler" here? Going by wiktionary conjugation tables - https://en.wiktionary.org/wiki/i%C3%A7mek - it should be "içerler".
Is the table wrong, or can you always use the 3rd person singular form instead of the 3rd person plural form (possibly in formal / informal settings), or is it just a special thing for this verb?
You can use both. If you use "içerler" you can omit "onlar". If you really want to know the specifics, it is "içer" when they do the action together, and "içerler" when everybody does it on their own - but that's just a grammar rule nobody thinks about when they speak.
As far as I have found out thus far, Turkish word order is: Subject_ Object_Verb. (S.O.V)