I will say it one more time, and if you don't get it, I am moving on.
There is a difference between:
#1. THE feast of unleavened bread
(Leviticus 23:5) vs.
#2. FIRST DAY of unleavened bread, when they killed the passover
(Mark 14:12).
If you were to read the whole of Leviticus 23:5-8, they ate unleavened bread ALL seven days (See: verse 6).
This means:
The 14th, they would have ate unleavened bread (Which is confirmed by Exodus 12:8). This would be the FIRST DAY of unleavened bread, which is the Passover.
The 15th would be THE feast day of unleavened bread.
It's kind of like Christmas Day and Christmas Eve.
The word "Eve" distinguishes the two. Just like the word "FIRST DAY" distinguishes itself from "THE Day of the Feast of Unleavened Bread."
As for your suggesting before that Matthew 26:17 is in error because it suggesting that the First Day of unleavened bread is a separate day (the 13th) from the Passover (the 14th) because they were going to prepare for the Passover:
Well, you cannot make the assumption that just because they are preparing for the Passover that they are not already within the time of the Passover.
In fact, Luke 22:7-9 says this:
7 "Then came the day of unleavened bread, when the passover must be killed.
8 And he sent Peter and John, saying, Go and prepare us the passover, that we may eat.
9 And they said unto him, Where wilt thou that we prepare? "
So unless you want to say that Luke 22:7-9 is also in error, Matthew 26:17 is saying the same thing here as Luke 22:7-9.