I still have 2 unplayed Dwarrowdelf APs. Do you recommend I play them multiplayer (2-player) first or play solo first? I know play experience is different between the two. Just want your opinions as to which mode is more enjoyable. Ive been playing just solo, until a month ago, my brother showed interest, so the option of playing 2-player came up.

It depends on your brother's personality and interest level. If he is willing to endure possible disaster and get more motivated to try again then you should go multiplayer and figure the quests out together. If he is a casual gamer type who is mostly interested because it is a chance to have a social activity with you then I suggest you play solo first to give yourself the best opportunity to make his experiance less painful. IMO

In my opinion, multiplayer is the best option. Moreover, if one of the APs is Foundations of Stone, it will be tons of fun in multiplayer, and just merely enjoyable in solo.