Kahga's (supposed) problem with the Thieflings isent that they are sitting on the land that they are or are not using per se. As fas as I can tell anyway. (might need to replay it. Its been a few days) Its more so that theyre beeing dragged in their conflict and (apparently) are eating their food. We already adressed the 2nd point but the first has some merit to it. To a certain degree.
The group of Goblins are the ones who are investigating the crashed ship. Its highly likely that they would have investigated the grove regardless if the thieflings were manning the ramparts or not. Infact theyre doing the defending for the druids in this case. So in a a sense they are helping the druids instead of beeing a drain on their supplies. If some died those could have been druids who died. Kahga doesent examine the situation outside of the grove though. She stays in the inner circle and just bosses her fellow druids around without even taking a look at the actual situation. The Goblins would have already been inhabbiting that ruined temple for a while, so the Thieflings have nothing to do with them 'suddenly' appearing.
It seems to me that alot of ingame conversations are still missing because alot of things arent talked about. Either things arent brought up or they dont really go into details of things like how Kahga got into her position.