That's correct Slip initial entrance visa is valid for a maximum 33 months it's just to give that person a 3 month window to initially enter UK so basically as long as Harlene has completed a minimum 30 months residency then she can apply for FLR so will put it like this:
Earliest Harlene can apply for FLR is 02/05/17 ie: maximum 28 days before her 30 month residency.
The latest is what it says on her visa 21/08/17 so hopefully that gives you time to sort things out and a bit of a window for manoeuvre.
As soon as your set with the requirements UKVI require and within the dates I have said then apply, it may be sooner than the latest expiry on her visa and at least another step onwards and upwards.
Wishing you the best of luck,
Mark...