Regarding the MS cooldown problem it is important to know if it starts when you cast it or when it fires.
If CD start when you cast Multi shot, with 5/5 in arcan shot cycle is :
0s Start MS
0.5s MS fires
1.5s GCD finished, AS
6.5s AS
10s Start MS
10.5s MS fires
11.5s GCD finished, AS
16.5s AS
20s Start MS
20.5s MS fires
etc
In this case, yes 5/5 arcan shot is perfect.
But, if the CD starts when MS fires :
0s Start MS
0.5s MS fires
1.5s GCD finish
6.5s AS
10.5s Start MS
11.5s AS is up but still under global CD
12s You can finally AS
So in this case you are wasting some talent points in AS.
Better solution if CD starts after MS fires is 4/5 in arcan shot :
0s Start MS
0.5s MS fires
1.5s AS
6.7s AS
10.5s Start MS
11s MS fires
11.9s AS is ready but still under GCD
12s AS
17.2 AS
21.5s Start MS
22s MS fires
22.4s AS is ready but fires still under GCD
22.5s AS
etc
In a way I prefer if the CD strat after MS fires, as It will save one point to put somewhere else in my build, and you have a 0.1s in each cycle to compensate for the lag.