Grace notes in Chopin op 63 no 2

[roncesvalles]

I'm currently learning Chopin's Mazurka, op 63 no. 2, and I'm having a bit of trouble in the middle section on the mordents.  For fluidity's sake, I've been fingering them 3-4-3, so I can get to the next notes with ease, but I am not playing them well.  When I play them they're either too slow or too loud/uneven...I can't consistently play them at the right dynamics and speed.

Should I just continue to practice the parts that are difficult for me in the hopes that I resolve them, or should I look for exercises to strengthen my fingers/reaction for these particular motions (like hold down 1,2,5 and try to trill 3,4)?  I don't want to develop a mental block for this part, where I mess up because I've conditioned myself to expect failure, and maybe it is best to approach this with exercises, for a more generalized solution that may help me in future pieces.  

[roncesvalles]

I'm getting better with the 3-4 mordents--a mixture of practice and exercises.

Also, in measure 20, on the third beat, there's a (  before the chord in the left hand.  I take it to mean that the chord is to be rolled, but the chord falls neatly in my rather large hands.  Is the rolling of the chord mandatory here (if it was I would think that it would have the squiggly line before it)?  Instinctively, it feels more consistent to me to play it as a chord than to arpeggiate it.