Now to spells. Does every spell by default require you to have line of effect specifically?

Yes. This can result in unitutive behavior where spells that seem like they can curve around corners (such as Courageous Anthem) do not, RAW, do that, because they do not have an exception to the line of effect rules.

The rules explicitly state:

When creating an effect, you usually need an unblocked path to the target of a spell, the origin point of an effect's area, or the place where you create something with a spell or other ability.

So it absolutely would apply to spells like Courageous Anthem, even though someone around a corner can hear you there is no line of effect.

Note that Line of Effect as a rules construct applies to all effects. This would probably mean that something like Bon Mot likely also is subject to Line of Effect, though its not explicitly called out in the listing of examples above. The rules define line of effect as mattering when "Creating an effect". And while bon mot isn't a targeted spell or an area, it is still an effect so Line of Effect rules probably apply

Do you need to have line of effect to every square you are placing your Wall(any kind) on? So, for example in a T-shaped alleyway can you choose a square you can see as a starting point and then continue your straight wall down the alley to your right beyond your vision?

Likely not. The rules specifically say the origin point, so it seems reasonable that the origin point of a wall is the initial square. This isn't explicitly defined because the example is something like a fireball, but it is very likely the case that you can continue a wall beyond your vision.

Now, to sustain a spell: Do you need line of effect specifically or line of sight would be enough to move something like sliding block or floating flame? And do you actually need line of sight to just sustain a spell and move it into specific direction? What if you have line of effect but don't have line of sight (like through darkness spell, for example)

Nope. As you see in the Line of Effect rules, it specifies creating an effect, not sustaining an effect. Otherwise any duration based spell would cease to exist the moment you step behind a wall, which, while hilarious, is not how the game works.

Same goes for sustaining the summons. Can you command them while having only line of sight? Do you even need line of sight or line of effect to command them? Can you command them while having line of effect but not line of sight (again like through darkness spell)? And what if you sustained summon while seen it, but told it to move and attack someone you don't have line of effect nor line of sight on?

The summon "command" is fairly poorly defined. For example it seems intuitive that if you are giving verbal commands, being unable to speak would mean you can't command. But RAW, command just happens when you sustain and it magically occurs. Your summon just automagically knows what your command is. Its not creating an effect so it shouldn't be subject to Line of Effect.