No. 'Come across' is to find, or to meet. 'If you come across my cousin, say hello'. Get across is to manage to cross over something. 'I will try to get across the river, but it is very deep'.

Nope. Get across = To convey. E.g. "I just don't know how to get it across to you." Come across = To seem like // appear to be E.g. "He came across as an angry and bitter man."