It is "öppet". Usually, when the "en" form ends in -n, the "ett" form will replace the -n for a -t.

You can check noun and adjective declension tables on Wiktionary: https://en.wiktionary.org/wiki/%C3%B6ppen#Swedish

Not quite, but you're on the right track. If the restaurants had been singular "the restaurant", the adjective would indeed have been öppen. But that means that the adjective agrees in number (singular) and grammatical gender (common), which in this case means it's öppen.

Adjectives can come in many forms, and only a handful have this pattern of -en/-et/-na in common/neuter/plural. The regular form is -- / -t / -a, meaning it's unchanged for common singular, get's a -t for neuter singular and -a for plurals.

Example: tokig ("crazy") is tokig/tokigt/tokiga in those three forms.

Does that help?

If you're on mobile, that's frequent to happen, so don't worry too much.