Umbral Mantle is better than Staff of Domination because it lets you win aggro battles with your small dorks
Well, active staff (with >4 elves and priest/archdruid) means gg in 99% of scenarios. I only lost to blazing archon.
With no priest/archdruid, or with less than 3 elves: Umbral mantle gives +2/+2 to an attacking creature. maybe +4/+4. Staff draws a card, or taps creatures (for example, bsc). I prefer staff.
With priest/archdruid and 3 elves in total: Umbral makes priest a 1001/1001 (hope your opponen does not have something with deathtouch). Staff just draws or taps creatures. I prefer umbral, but it's still blockable, not instant win in a good number of cases.
With priest/archdruid and 4 elves in total. Umbral makes all your creatues 1001/1001. Staff just draws or taps creatures. Here is where umbral shines, because blocking 4 or more creatures is not as easy as blocking 1. However they shouldn't have summoning sickness! I of course prefer umbral mantle
With priest/archdruid and 5 elves or more in total. Umbral makes all your creatues 1001/1001.However they shouldn't have summoning sickness! Staff draws all your deck if you want to, gives you infinites lives if you need to, allows you to tap all your opponent creatures. Playing emrakul, in my eyes this is safer. Win more? Well, safer.
I'm not discarding mantle at all. In fact, mantle is a little quicker than staff, allowing:
T1: forest + llanowar
T2: forest + priest + elf
T3-A: umbral mantle. At that point priest attacks for 890389083
T3-B: forest + elf + umbral mantle. At that point 3 creatures attack for 890389083
With staff it's not reasonable to have 5 elves by turn 3 AND have the mana to untap priest.
About the list, I like a lot Mirror entity, but if you are not playing any other white spells I won't play mirror, specially if you are not playing caverns (with caverns and birchlore+gsz it's another call).
you play 2 behemot+1 mirror entity +2 weird harvest to use huge mana. I play 3 genesis wave +1 emrakul. I'd probably try weird harvest, but I'm a bit afraid to fetch snapcasters for my opponents. Genesis wave has been great in some tournaments but meh in my last one. Genesis wave with less than 6 mana is stupid. With 6-7 mana is a bit desperate. With 8-9 mana most of the time gives a good advantage, but not enough to close the deal. For 10-17 mana you should won in the spot, or maybe the next turn. With 18 or more i lost once, and because I missplayed it (I put emrakul into play, not winning the extra turn, instead shuffling it in my deck and fetching it with fauna shaman).
How about playing 8 priests and 3/4 umbral/staffs? It makes the deck more quick imho.