Its good to know that keeping the number of adversaries in check can atleast let the heros work... are you talking like some of the house rules to reduce monster hitpoints so things dont drag so much (because the party lacks strikers?).
Does increasing the number of adversaries make things too dangerous because the dice make it easy for things to be swingy, even if I build say a level 3 encounter... where attacks against the pc's will miss the majority of the time in a particular encounter they might get a set of bad luck.. and the pc's will tend to be reliable in hitting but dont get enough attacks to be really dangerous compared to the number of adversaries they are against?
Usually "Swinginess" doesn't happen as much with 4e. A crit may bloody you, but that's it. Wardens in particular look very sturdy to me - more HP than fighters, and the dual saving throws let them kick many conditions in the teeth.
The thing you need to look out for are high defenses. Soldiers are going to slow things down (because soldiers have high def and low damage output). Brutes do the opposite - low chance of hitting, low def, high hit points and damage output.
Honestly, just play it by ear. Experiment. Use 3/4ths of a monster's hit points, and see how that works. If it's too fast or too slow, adjust.
hmmm these druids definitely leaders of their people... like the classic druidics...I have a place where nature spirits dying would be quite likely I do like your descriptors of the side-effects... perhaps we need some purification ritual to help out an area whose spirit is not dead just tainted and injured.
There are other things you can play with, too.
The nature spirit could be very unhappy. To use a river spirit again, maybe the locals who fish or travel up and down aren't giving respect to the spirit like they used to, or they're polluting. Or the spirit could be greedy - it wants more tribute, more power, sacrifices, etc - and the locals aren't willing to give that.
Maybe some humanoids have started to worship the river spirit, and are giving animal/human sacrifices in exchange for hampering the locals' travels/fishing - the spirit takes the sacrifices, and starts hampering the locals. In this situation, the river spirit is just siding with who is giving it more power; it has no morals and no loyalties, so from its perspective it's doing nothing wrong.
So the PCs' job is to go knock some sense into the offending party.
Or, the people who use the river want to expand the river - dig more. This is good for the River spirit. But for the forest or earth spirit, whose territory is getting dug up and flooded, isn't happy with this development. So the duo could be sent to broker a deal between Spirits and locals.
The PCs could be tasked with an escort mission. Perhaps there is a sacred ram that the Nature Priests plan on sacrificing during a great celebration - and someone has to bring the sacred ram to headquarters. The PCs are tasked with escorting the ram - and of course, things want to attack the ram - hungry monsters, enemies of the druids who want to disrupt the ceremony, etc.