Yeah, because we all know that when most people screw up, their first action is to own up to it fully, rather than laying the blame on someone else?

The lengths you go through to defend Fiona are matched only by the lengths you go through to condemn someone like Meredith. I don't see the gifs flying out to defend her asinine decisions, and I don't see he walls of, "It's not her fault," to make her choices acceptable. Perhaps one needs to be a magical, female elf to be excuse for being inane.
But her leadership over the Mage Rebellion was tenuous at best, since even she later admits that her title of Grand Enchanter means little since they left the Circle? The only reason was was "in charge" was because everyone wanted her to be, so she could have been replaced at any time? The fact that Alexius was so easily able to depose her and take over with little opposition kinda suggests that her leadership wasn't backed up with anything save public opinion and she had no way to actually hold onto her power if challenge?
Alexius didn't "depose" her. He offered to protect all the mages and said that a Templar attack was imminent. Fiona, the sly fox she is, pounced on that offer as if it were a mouse. The reason she was able to do that? Because she was the de facto leader of the Mage Rebellion. She isn't some figurehead for the public.
After all, if it was so easy to oppose the Venatori and the alliance, why didn't any of the rebels rise up against them and tell them that the deal was off? Perhaps they did in one timeline, but with his ability to go back in time, any potential dissenter is probably dealt with swiftly, before any uprising is able to get much traction? The mages couldn't pull a fast one on the Venatori when they can Ctrl-Z at will?
Because they can't "undo" on a whim until it works. That doesn't make any sense. Fact is, it's not easy to oppose the Venatori. Especially after you let them INSIDE the fortified castle. That kind of makes the fortifications pointless. Before that? They were virtually no thread, especially with the rebel mages backing the arl's troops. Of course this goes back to the hoop jumping I mentioned earlier, so it's not surprising to be seen here.
You also realise that the Mages aren't behind the castle walls, they're in the village which was where they were allowed to stay, as was the original agreement with the ruler of Ferelden? The only reason the castle was taken was when Alexius threw out Arl Teagan... but even then, the Venatori control the castle, not the Rebellion? If the Templars invaded, the Venatori would be safe, the Rebellion would be crushed?
What are you talking about? You do realize what a castle is, yes? Redcliffe Castle, as in the location, is a keep. Redcliffe Village is a proper walled castle/city, complete with fortifications, groveling peasants, and tactical advantages. This of course all on top of the save harbor of all of Ferelden. There is literally no excuse for the stupidity shown by the rebellion, and Fiona, here.