Yes. As long as you qualify as a legal target for the spell, you can choose yourself as the target.
Yes, tap abilities are instant in Magic: The Gathering.
Yes, in Magic: The Gathering, you can activate creature abilities at instant speed during a game.
Yes, you can tap artifacts at instant speed during your opponent's turn in Magic: The Gathering.
Yes, planeswalkers can attack during combat in Magic: The Gathering if they have the ability to do so.
Yes, players can choose to attack planeswalkers during a game of Magic: The Gathering.
No, Bloodrush is not an instant card in Magic: The Gathering. It is a mechanic that allows you to discard a creature card to give another creature a temporary power and toughness boost.
'Flash Counter' is a Blue Instant costing 1U. "Counter target instant spell."
Enchantment abilities in Magic: The Gathering can be activated at any time you could cast an instant spell, unless stated otherwise on the card.
In Magic: The Gathering, instant cards can be played at any time, even during your opponent's turn. They can be used to respond to your opponent's actions or to create strategic advantages. Instant cards are typically played by paying their mana cost and following any additional instructions on the card.
No, unless they have haste.
The official ruling on Nivmagus Elemental in Magic: The Gathering is that it can exile instant and sorcery spells you cast to grow stronger, but it cannot exile itself.
In Magic: The Gathering, creatures cannot attack or use their abilities the turn they are summoned unless they have haste. This is known as summoning sickness.