I doubt that those three objectives are particularly compatible. Wars are typically started by spying and destroying the sacred symbol of a religion which also happens to be the main dwelling of the settlement.
Spying is generally performed by being innocuous and opportunistic, which will get you into wars if you are caught and will make hard objectives, like tree killing, awkward.
Killing the sacred world-tree will likely start a war and raise everyone's suspicions, particularly about that exotic group that just turned up.
The odds of pulling off all three are... low...
Hrmm, to sway suspicion you would need familiarity, that means getting jobs and staying there for weeks if not months, or at the very least being called away on urgent business that was not brought to your attention by your own group.
If you want to avoid a war then you absolutely must not leave enough information to be identified and then leave on the same night that the tree is assassinated. That is pretty much an automatic war scenario.
Probably the easiest way to gain information is to insinuate yourself in the regular traffic, sign on with traders or something. Ideally the agent will not know about your nefarious plan, given that the scouting party is aware of the plan, that would mean having a third party surreptitiously gather information.
...
Or you could just walk up to them and ask to kill their tree, it really might work...