The best opportunity to kill Karvelos is near the memorial; it's also the worst. There's a high probability that our people were intercepted by unknown individuals working for our employer, and they also know about the planned assassination.
a) These could be other stone hunters. In this case, they will try to intercept Karvelos before us or claim the stone after we do all the work for them.
b) These could be Ninsharr from the Sunset or any other pursuers; we have plenty of those. Here, the situation is reversed: our squad will be the target of the attack.
c) These could be other unknown individuals who are helping Karvelos in some way.
The first option seems the most likely; the second is also possible, but if they were bounty hunters, they would have acted differently, in my opinion; the third possibility exists, but it seems that the unknown individuals have not contacted Karvelos, so it is unlikely that they are his allies.
Therefore, in the current situation, I propose three options:
1) Attack near the memorial. The advantages: it's the easiest way to assassinate Karvelos, there's little security, and it will be less noisy. The disadvantages: it's likely that the third party knows about this and anything could happen. Of course, before the attack, we need to leave scouts there and carefully prepare for an ambush, rather than just come and start hacking at the Ilthani.
2) Move the attack location. This is more difficult, but still a viable option. The advantages: we will likely avoid a possible ambush near the memorial; the disadvantages: it will be more difficult to kill Karvelos himself, there will be more guards, etc.
3) Don't attack at all.