1. It should be alright, check this map.Your hotel is located at Guldgränd.
When going to the ferries you be observant of signs pointing you in the right direction. https://mitt.sl.se/ficktid/karta/vinter/Södermalmstorg_term.pdf
2. It's fairly easy. The subway platform you should use (southbound red and green line towards Ropsten, Mörby centrum, Farsta strand, Skärpnäck and Hagsätra), is 3 levels down (see map below).
Arlanda Express will be fine. Regional trains are more scenic if you are fond of train tunnels very far down below surface level. Arlanda Express will arrive at Vasagatan in this map- https://mitt.sl.se/ficktid/karta/vinter/city_stn.pdf
3. Agree.
4. Only Djurgårdsfärjan (#82) stops at Slussen. The ferry to Frihmanen (if that is where your ship docks) departs from Nybroviken.