"Do I NEED to use hashtags here?"
Short version: No.
Long version: CounterSocial does not have any algorithms to direct people towards the content they like or spare them from the content they don't. This means that hashtags are a considerate way to help other people to curate their own experience.
Columns can be configured to show or omit chosen hashtags, which makes hashtags important, but not required. #CoSoTips
"Do hashtags automatically apply to replies?"
No. Hashtags only apply to the hashtagged post, however your replies will only show up for those who are included in the conversation.
Everyone else would have to click on the thread to see replies, which they'd never see if their hashtag preferences have filtered it out.
@CoSoTips what if it's mellowing my harsh?