Wow... that is one very buggy party. I remember it being a little buggy ages ago but I've had to jump through hoops to make the party a success. I found that pausing the game long and often sorted most of the bugs where goals would not register despite the sims having completed them. At one point I paused for at least 35 seconds while I waited for the violin playing to register. It did and then all the sims got stuck around the fire, so I paused the game again this time for over a min and when I pressed play all the sims reset and ended up jumping to one side of the fire. Ugh!
I found the best way is to have as few guests as possible that are outside your own household, always a good strategy for parties anyway, and to have really long pauses after your sims complete a goal.
Not ideal.
To get the play a violin/guitar to register I clicked on the fire while the instrument was in the hosts inventory. He played until the queued action expired and then I hit pause. After I hit play, the goal registered. In some games the 'Play a violin/guitar' requirement came right back... such a pain all around really.
I also noticed that some goals such as 'tell a story' just didn't get the correct animations, instead all the guests just stood with their heads nodding and yet the goal completed anyway. (Although the sims didn't get the moodlet that they would normally have got for listening to a story.)
Don't know if this helps you really but it is certainly a lot of hard work getting this party to function semi-normally.