No No No Sheep
1) Yes, Ancient Mathom is a must-have (imo) in any spirit deck (well if the scenario is all about combat, then maybe no, but is really powerful).
2) I don't understand this question. The use is the same of any location, with the difference that it'll go to the enconuter deck when it is explored, so it makes the scenario easier, because you know what'll be the next card. You can exploit this to advance freely in a solo play.
3) In the card it doesn't say anything about "1 round" or "2 phases". So its effect lasts forever. Forever = until the enemy leaves play in any possible form.
wow forest snare is a bit overpowered imho.. i though it last only 1 round