話がすっきり >>What does this means in this context?
2 girl are talking about MC, who was arrested by police. The police suspecting MC is the murderer of a crime.
G1: 無実なのだから、そのうち帰してもらえるのではないか
G2: 甘い。警察なんて、無理矢理にでも証拠をでっち上げるぐらいするわよ.
てか、そもそも本当にあいつじゃないの？それなら話がすっきりするわ
If you can please also give me English translation of the last sentence
すっきり is a mimetic word that describes how clean or tidy something is. Depending on the context and the subject, すっきりする can mean "to be clear/clean/tidy/simple", "to feel refreshed", etc.
てか、そもそも本当にあいつじゃないの？
Um, maybe (the murderer) is actually him in the first place?
それなら話がすっきりするわ
[literally] If that (is true), the story would be clear/simple.
That would clear things up.
(Of course, this is probably a dirty joke. Is Girl 2 a tsundere character?)