The problem is the wording of fast hand. It allows to use a magic item that requires the action. Scrolls don't require a magic action.
The allow you to cast a spell. Those spells may have a casting time of one action.
You are ignoring the question.
If reading the scroll is not part of the magic action, why can't I read it and then cast it days later?
I read the ability and did not try to find a connection that is not there.
Words matter:
"... you can read the scroll and cast the spell using its normal casting time ..."
It does not say you read the scroll and then at some random time you want you use the normal casting time to cast it. It has the word "AND" reading the scroll is done as part of this.
This is crystal clear in the wording, otherwise how do you explain the meaning of the 14 words above. They are not two distinct things.
You can. Hold it for one round using concentration.
Where do you get this concentration from? Nowhere does it say you read a scroll and then concentrate on it until you cast it.
Also nowhere does it say you read it and then can use a ready action to cast the scroll. The wording in the PHB would actually prohibit this as it says
"the normal casting time".. While using a reaction to cast a spell with a magic action casting time is allowed generally a reaction is not the
"normal" casting time for such a spell.
You are just making new rules up to try to defend your position when it is clear how the mechanics of using a scroll work.
I really don't know what you want to prove with your example.
I want to prove it is obvious what the part on the spell scroll means and if I am to accept your interpretation that reading it is not part of the action used to cast it then you can read it any time and cast it at any later time. No concentration, no ready action, multiple PCs can read it and they can all cast it days, weeks or months later .... because why wouldn't they be able to if reading it is not part of the same action used to cast it.
You refuse to engage in this and are making up new things (like concentration) to defend your otherwise indefensible position.