Outcaster Trailblazer’s first ability isn’t a mana ability. It uses the stack and can be responded to.

If one or more static abilities that apply to a creature entering the battlefield change its power, those abilities are considered when determining whether or not Outcaster Trailblazer’s second ability triggers. The same is true for replacement effects that apply to it, such as entering the battlefield with one or more +1/+1 counters or entering the battlefield as a copy of another creature.

Once Outcaster Trailblazer’s second ability has triggered, lowering the power of the creature or removing it from the battlefield won’t stop you from drawing a card.

Plot abilities are written “Plot [cost],” which means “Any time you have priority during your main phase while the stack is empty, you may pay [cost] and exile this card from your hand. It becomes plotted.”

Exiling a card using its plot ability is a special action. Once you announce you’re taking that action, no other player can respond by trying to remove that card from your hand.

You can’t cast a plotted card on the same turn it became plotted. On any future turn, you may cast that card from exile without paying its mana cost during your main phase while the stack is empty.

If you’re casting a plotted card from exile without paying its mana cost, you can’t choose to cast it for any other alternative costs. You can, however, pay additional costs, such as kicker costs. If the plotted card has any mandatory additional costs, those must still be paid to cast the spell.

If a plotted card has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.