Specifically regarding mercury – wouldn’t it be too much for the Orcs to get so many resources at the start? The mercury mine was deliberately placed further away to somehow control the number of Cyclops that can be produced as early as the first week.
Hint: build a warehouse, and you’ll have mercury.
Regarding overabundance – it’s better to have too much than too little.
And seriously, the number of monsters is high because players start with developed cities, plus there are many external structures for hiring. If there were fewer monsters, the map would be cleared too quickly.
Added 3 minutes later
By the way, of all three factions, the Orc start is the most difficult. Be prepared to defend against demons around the end of the third week. However, in the long run, it’s easiest to play as Orcs in the later stages of the game.
In general, I’ve played this map several times with all three factions; it’s just easier to play as some, and more difficult as others.