Turkey has 36 million Tourists annually with a big portion muslims plus it has one of the biggest borders with Syria, before the wall was built it was hard to controll it now its widely shut down thanks to the wall on most critical sections.
And now to Ammonium Nitrate, as you posted those have been used as fertilizer, pkk is still using those in their bob attack does it mean it supports pkk?
Those are not evidences but a bunch of assumptions.
That's actually evidence that all those foreign jihadists came to Syria through Turkish borders, that's not an assumption but evidence. Now you can say that Turkish officials had no clue that was going on and couldn't secure the border, that's a different discussion that goes more in depth though that's hard to believe.
As for states and evidence. It's easy to circle around evidence and name it an accusation/assumption without any proof, especially when it comes to politics given that officials has so many people working underneath them. Whatever is shown you and anyone can easily shrug it off as accusations and assumptions. This is why there's judges that decide, a side on state level will barely ever admit to something even when there's enough evidence to suggest it.
Before 2013 the FSA also used to work with ISI (pre-ISIS), in 2013 they took Raqqa, here again the FSA shared weapons with them prior to the takeover. Don't you think that co-operation against Assad was reason enough for Turkey not to stop foreigners entering Syria, in other words they have pulled a blind eye to that for years.