No, it's not true. You get only one 'Normal Summon' per turn, but not only can other cards 'Special Summon' one or more monsters in the same turn as you do this, but other cards can let you Normal Summon more than once too.
You are only allowed 11 cards as a maximum on a field, you have five monster card slots, five spell/trap slots, and a Field Spell Card slot.
The only time you can do this is if you are tributing the monster for a tribute summon, or sending them for some kind of special summon, like for Destiny Hero Plasma or a Synchro Summon. But if you wanted to summon another Lv4 monster, you cannot simply make room by sending a monster from the field to the graveyard, you can only do so by effect or game mechanic, like when it is destroyed by battle.
Magical hats can only be used during your opponent's battle phase. Seeing as they can't flip their monsters during their battle phase, no. However, if your opponent does activate a spell or trap card during the battle phase, you can use Magical hats and protect your monsters. Overall, yes. You can use magical hats to protect your monsters from targeting effects as long as it's during your opponent's battle phase.
Only if it has an effect that lets it attack more than once per turn, like Asura Priest does.
You cannot manually. Such an occurence could only result from a card effect.
No it lets you bring back a monster from the graveyard and place on the field and stays till its destroyed
The card "Zombie World" is only available in the Zombie World structure deck.
If you're talking about monster zone 'columns', then these have nothing to do with attack declarations. A monster on one player's side of the field is able to attack the monsters on the opponent's field, regardless of which column they are in.
You have five monster slots on your Field, so even a card that can summon an unspecified amount of monsters (like Return from the Different Dimension) you will still only be able to summon a maximum of 5 to your own side of the field. There are no cards that can increase this, and Trap Monsters take up a monster card slot so it's not like you could have five regular monsters and an Embodiment of Apophis.
In general, yes. Continuous effects are only applied while the monster is on the field, an activated effects (Ignition, Trigger and Quick) can only activate when the monster is on the field. If they activate in other zones (Honest, while in hand, Zeta Reticulant, while in the graveyard, DD Survivor, while out of play) then it will specifically say so on the card.
No.You can put monsters in the graveyard only when your monsters are defeated.
As long as the effect actually does target, and only targets one face-up Dark monster, then Obsidian Dragon can protect itself when targeted by it, whether or not it is the only monster on your side of the field. It cannot protect itself from an effect that targets two monsters, even if only one is Dark. It cannot protect itself from a non-targeting effect like Lightning Vortex, even if Obsidian Dragon is the only monster on that side of the field.