The Bonus Spells slots for higher ability score is only additional spell slots once you reach a Wizard Level high enough to personally cast any spell of that level.
Absolutely, if you are a Level 1 Wizard, you can only prepare level 1 spells.
You cannot prepare any Level 2 Wizard Spells till you reach Wizard 3.
It is obviously absurd that a Level 1 Wizard with INT 20 (IntMod +5) can prepare and cast a level 5 spell. I don't think you would like a Level 1 enemy Wizard to have such capability against you.
"Suppose at the end of his first adventure the wizard find a scroll
While it seems a Wizard 1 can copy it into his spellbook by following all the appropriate checks, he cannot prepare it until he becomes a Level 5 Wizard as that is the level that a Level 3 spell becomes possible for Wizard to prepare.
A lower level wizard may directly cast a scroll of a higher caster level than his own, by making a caster level check:
1d20 + your caster level vs a DC of 1 + Scroll's caster level
If the Fireball Scroll was scribed at the minimum caster level of CL5 then you have an 85% chance of activating the scroll.
This of course will remove the spell from the scroll.
suppose the wizard reaches lv. 2: can he choose, as new spells to
write on his spellbook, one 4th-level spell and one 5th-level spell
From the source that states how Wizard gets new spells:
"Each time a character attains a new wizard level, he gains two spells
of his choice to add to his spellbook. The two free spells must be of
spell levels he can cast."
Even if a Level 1 or Level 2 Wizard were to somehow end up with even a 2nd level Spell in their spellbook they could not prepare it and count not cast it.
Level 2 spells = Prepare From Wizard Level 3
Level 3 spells = Prepare From Wizard 5
Level 4 spells = Prepare From Wizard 7
Level 5 spells = Prepare From Wizard 9
Level 6 spells = Prepare From Wizard 11
Level 7 spells = Prepare From Wizard 13
Level 8 spells = Prepare From Wizard 15
Level 9 spells = Prepare From Wizard 17
There are no viable shortcuts to skipping all the vastly under-appreciated lower level spells. Not that fireball will help your overall successes anywhere near as much as a level 3 spell like Haste would.
See Sacred Geometry get more out of lower level spells.