I have asked How to get cold air from one room to another now I am thinking, the AC will be in the bedroom but what if I set up a dehumidifier (perhaps with a pump) in the kitchen connected directly into the drain? This last week mid day we had 60-70% humidity with 30 C temperatures and 90% humidity during the night although only 20C temperature outside. From what I read, it seems portable ACs struggle in high humidity to keep the air cold, perhaps this would help? Or it's taking air from the outside anyways and this doesn't matter?
Dryer air does contain less heat energy, and therefore would cool more readily. The real benefit, though, is in comfort. 74 degree air at 50% humidity cools skin more rapidly and feels much better than air at 80% humidity. In fact, you may find that you can tolerate a higher temperature if you have drier air. Your home will also smell better and you'll have much less mildew.
A dehumidifier is essentially just an air conditioner that re-warms the air before blowing it back into the room. If your goal is to cool the room the additional step of re-warming the air is superfluous. When water condenses it releases heat, so the cooling coils in a dehumidifier absorb that heat. It results in cool water going down the drain. The air that comes out of a humidifier is slightly warmer than the air that goes in. Energy will be spent removing humidity which the a/c will have to make up for in re-cooling the air. It will always end up as a net energy loss.
Before the edit I suggested an indoor a/c. I know they make indoor air conditioners, but the idea of an a/c is to move heat from indoors to outdoors, so I really don't know how an indoor a/c functions. The only possible way I can imagine them working is by evaporating water. Since your environment is humid it will already be pretty close to saturation, so that method would be extremely inefficient.
It sounds like your real problem is circulation, since you only have one window available. But I don't think a dehumidifier is going to do anything for you. The air conditioner itself is the best dehumidifier you can get.