Would they want 5th freedom on the outbound leg? I assume that inbound to Turkey they will go non stop?
THY will undoubtedly be hoping that this will be a short-term issue only - weeks rather than years. So they're unlikely to be giving any thoughts to fifth freedom sales at this stage, especially on a one-way basis.
during the day capacity would be an issue there is a severe lack of wide-bodied stands at MAN
I haven't checked the THY timetable for specifics but would not expect a problem with this. Transatlantic flights departing from IST are highly unlikely to clash with MAN's overnight and early-morning stand congestion. Late morning and early afternoon timings should represent no problem at all for stands.