Barcelona wasn't in their good form, but it's a big joke if they're lose against Shakhtar Donetsk and Royal Antwerp because both of them are in different level. If Manchester United was in this group, I believe Manchester United will able to qualify for knockout stage.
Either Barcelona or Napoli win the match, their next match is against Arsenal which I think they would lose.
In the UCL the first round will be in Napoli, so losing it is probably realistic. But the best thing is at least they don't lose by more than 1 goal. I also still feel confident that Barcelona could have drawn in the first round. So they have points for the 2nd game at Camp Nou. Besides, the game is still February. There is still a transfer break that can be taken advantage of. For Napoli and Barcelona I guess Barcelona will win and continue the round of 8 big teams.
I think the performance of the teams will determine if Napoli can take the lead at home in the first leg, Napoli will have the home advantage over Barcelona since they will be playing in front of their fans but it will take good performance to beat Barcelona regardless of their poor performance lately in their league La Liga. I think Barcelona losing to Napoli in the first leg is possible but just as you earlier said, if Barcelona don't lose with more than 1 goal then they will have a good opportunity to win Napoli in the returning leg to move to the round of 8 but since both teams are still struggling to be among the top 4 in their different leagues it's going to be an opportunity for them to push to see if they can win the UCL to get authomatic qualification for UCL next season.