Yes. One of the few times when you can voluntarily destroy one of your own cards, is when you wish to replace your Field Spell Card with either another active one, or even a Set one.
Yes, there can only be one active at any one time. If you already have a Field Spell active, you can replace it with another, the first is destroyed when you activate the second. If the opponent has one, then it is destroyed when you Field Spell card resolves. Field Spell cards can be set while another is active. Since the rule only affects active Field Spells, set cards are not destroyed by your opponent setting or activating a Field Spell card.
there is no limit
You may replace your active Field Spell card with a new one, either Setting it or Activating it. Your old card is destroyed and goes to the graveyard. Replacing an active Geartown by Setting a second one is a focal combo in Geartown decks. If your opponent has a Field Spell card active, this is destroyed when you resolve a Field Spell card. However their active one is not destroyed if you just Set one, and neither is their Set one destroyed if you activate or Set one too.
You can't ever just discard cards for no reason, you can only do so if instructed to by a card effect or a game rule, such as the one that lets you replace one Field Spell Card with another.
No. If Field Barrier is on the field, the Field Spell Card will be unaffected by the effect of Heavy Storm.Because Heavy Storm destroys all Spell and Trap Cards simultaneously, Field Barrier acts as a shield for the Field Spell Card, preventing it from being destroyed.To destroy the Field Spell Card, the Player must destroy Field Barrier FIRST. Effects that supposedly destroy both at the same time, such as Heavy Storm and Harpie's Feather Duster, do not affect the Field Spell Card.
Yes, they can. You can also play another field spell to remove the one that is currently in effect.
Yes, there can only be one active at any one time. If you already have a Field Spell active, you can replace it with another, the first is destroyed when you activate the second. If the opponent has one, then it is destroyed when you Field Spell card resolves. Field Spell cards can be set while another is active. Since the rule only affects active Field Spells, set cards are not destroyed by your opponent setting or activating a Field Spell card.
A Field Spell Card is destroyed when another Field Spell Card is activated or via an effect, such as Dust Tornado. If you activate a Field Spell Card while there is another already active on the field, your Field Spell Card will destroy and replace the existing one.
there is no limit
no you would have to use the card to destroy it when it is summoned because its effect activates as soon as it hits the field
No because the field spell card is just simply being replacing by another field spell. So, you can't use Magical Citadel of Endymion's effect to stop it from being sent to the graveyard. However, you may protect your field spell card with the spell card Field Barrier.
You may replace your active Field Spell card with a new one, either Setting it or Activating it. Your old card is destroyed and goes to the graveyard. Replacing an active Geartown by Setting a second one is a focal combo in Geartown decks. If your opponent has a Field Spell card active, this is destroyed when you resolve a Field Spell card. However their active one is not destroyed if you just Set one, and neither is their Set one destroyed if you activate or Set one too.
You can't ever just discard cards for no reason, you can only do so if instructed to by a card effect or a game rule, such as the one that lets you replace one Field Spell Card with another.
Your opponent will be able to destroy one of your Crystal Beast Amber Mammoths in battle even if two or more are on the field. If you redirect a attack to an Amber Mammoth, the conditions to activate another Amber Mammoth's effect would be gone because your opponent finished declaring an attack against a Crystal Beast Monster you control.
There is an important distinction between 'activating a spell/trap' and 'activating the effect of a spell/trap'. Light Barrier's activation was when it was placed face-up on the field, and this is the only time an activation negation card like Magic Jammer could be used against it. Then after they have resolved to the field, some of them have a reusable effect, this is not activating the card, just activating the effect.Later on, even if it has an effect that uses the chain (Light Barrier's does), this is only activating the 'effect' of Light Barrier, he is not 'activating Light Barrier', therefore Magic Jammer can not be used at this time.
'Activating a card' means to either place a Spell Card on the field, or Flip a Spell or Trap card face-up from a Set position.Some continuous Spells or Traps have a reusable, activated effect, such as Royal Oppression. When these effects are used, it is called 'activating the effect' of that card.
No. A card cannot be activated, if it is unable to resolve. Cards like Statue of the Wicked and Ojamagic, do not have a resolvable effect that can occur from activating them on the field. All their effects are triggers, which activate under other circumstances. Because nothing would resolve, they can't even be activated on the field. Ojamagic's effect only triggers after it leaves the field, so there'd still be no resolvable effect from trying to activate it on the field.