Yes, but only if you've unlocked Safe Mode for that stage.

Depend on exploration stars.. if it says you have to get for example na ocean se sector 96 wins.. I wouldnt skip them.. you can change ingame inmap the exploration stuff or in the wiki https://azurlane.koumakan.jp/List_of_Operation_Siren_Zones

There's no hard mode in Operation Siren. I believe FaturtlePlay is talking the regular campaign/event maps.

Mm yes prob but i think he means when he made the zone safe by clearing all once then returning prob in search of the collection memories for example where you have to keep returning to a zone till the data appears