3.x Spell casting and holding a shield question

[Doctor Awkward]

Unless you put your weapon in your shield hand for a second so you can cast.

A move action, which results in your character no longer threatening squares around them, stopping them from making attacks of opportunity and providing your allies with a flanking bonus.

Or sheathe it.

Not only does this provoke an attack of opportunity, afterwards which you are left in the exact same disadvantage as the previous option.

Casting is a standard action. Not like you don't have a move action to spare. And next turn you an draw as you move and also attack.

You likely don't.
You'd be dead from all the opportunity attacks you'd be provoking.

Requiring a free hand to cast and a shield occupying a hand is something very much intended and understood by the designers from early on.

And I'm sure you have some citation rendering this statement as something other than baseless speculation.

dura lex sed lex.

It's your opinion that the designers deliberately rendered the most iconic fantasy combat style ever to be a trap?

Take Somatic Weaponry feat.

Of those four classes, only cleric has Spellcraft as a class skill. The rest will not be casting spells until level 9.

Wizards have Daggerspell Stance which require using daggers in both hands. This would prevent them from casting spell to unless they take Somatic Weaponry feat.

No, it wouldn't.
The Daggerspell Stance spell doesn't have any somatic components. Quite intentionally, I imagine.

Although that does bring up the interesting point that the Ranger Two-Weapon Combat Style class feature is also a trap.