There are options for this as well
Yes, counters stay on manifested creatures, including ones created by manifest dread
Theres a pretty wide range for how to make a proxy. Some people write the name and abilities on a piece of paper covering a reversed card in a sleeve, some write the name of a card on the back of a bulk card like a basic land, and many people just print out the card in question
Since Flamethrower normally does 60 damage, now it has been increased to 120 damage
I can picture this card seeing play across multiple formats, possibly even being the big chase card for the entire set
Mana value is fixed and based only on whats printed in the top right corner of the card, not on how much you actually spend to cast it