I'm going to answer these by numbering the answers in accordance with the number of the question
1. There are two ways to go about doing this. First, you can place the outer shell of the house/building making it a "mesh" rather than a "static mesh" (so that it is hallow and the player can get through it) or you can make it a "static walkable mesh" (which can be found in the mesh properties after you've placed it "CLT:" then set to "static walkable," however this only works for some meshes) and build the interior piece by piece within the confines of the exterior mesh. You can find things such as place-able walls and floors in the asset browser for certain buildings like castles and a few smaller buildings such as inns. This however, is extremely confining as you can't let anything overlap and you must work inside the exterior mesh, so, the second method I have used addresses that issue. You can also separate them by instances. For example; say you want to use the large castle mesh and give it an interior but don't want to have to build it inside the actual mesh. What you can door is use a trigger near the castle so that once the player enters it, it teleports him to the interior, which you can build anywhere and simply hide it when it's not in use using the "hide layer" function in the quest editor. (I can explain this more thoroughly if you would like, but I need to move on now).
2. In order to put NPCs on your map you must use the community function. It's really a pain in the butt to use, but it allows for some very cool functionality to your NPCs. To use the community you must first place an actionpoint for your NPC. This defines where he will be and what action he will be doing. They can be found under "templates/environment/actionpoints. The entity things are the actual actionpoints that will be placed on the map, the job trees simply show what the animation looks like. After you have placed an actionpoint, go to the properties panel and give it a tag, then go to your asset browser and create a community (right click in any directory, though I suggest creating your own so you can easily find it, and click "create community"). Now name and open your community, then right click by the number that appears at the far left and select "add element." repeat this for all the greyed out areas on the line (It is VERY IMPORTANT you add all the elements first, otherwise it can cause REDkit to crash). Now find the desired npc and select him in the asset browser and return to the community editor and click the box under "entity template" and click the little green arrow. His name should appear there. Now select an appearence for him and leave the rest blank until you reach "timetable name." You can put any name here. Lastly, check the little boxes under "start in action point" and "always spawned." Now go to the top of the editor and select the "timetables" tab and add all those elements. Name your timetable exactly what it was in the other tab, then select the layer you put your action point for you character in and click the green arrow under "layer name." the last three boxes are pretty self explanatory, but the category for each actionpoint can be found by right clicking its "job tree" it will say what type it is in the bottom left hand of the window.
3. In order to place object, simply find the desired mesh/entity in the asset browser and drag it into the world (if it is a mesh and not an entity, it will ask if you want to drop it as a "mesh", "static mesh", or "impostor mesh"; a mesh is a 3D mesh, but with no collision, so it is simply hollow, a static mesh is 3D and includes collision, so the player will run into it rather than through it, and an impostor makes the mesh 2D). Make sure it is touching the tile or another static mesh, or it won't register.
I know this all seems overwhelming, but take it slowly and break it down step by step, focusing on why you are doing each step and it will make a lot more sense and become almost second nature, trust me (I've been working with REDkit for over a year now, so I've learned a lot and am happy to help others out in any way I can, and if there's something I don't know I'll find someone who does for you.) I wish you luck with your modding and if theres anything else you need at all, just reply here or post somewhere; I'll find it eventually, or message me! Hope I helped! :
