TiledMap has tilesetArray, from where one can read different tilesets, including external, but if several tilesets are wanted in one layer, one needs to merge them. I have some big old snippet for this:

			var tileMap: TiledMap = new TiledMap(Xml.parse(tmxFile), "assets/data/");
			//mapColliders = new FlxGroup();
					
			var tileSize: FlxPoint = FlxPoint.get(tileMap.tileWidth, tileMap.tileHeight);
			
			// merge tilesets into one
			var tilesetTileFrames: Array<FlxTileFrames> = new Array<FlxTileFrames>();

			var tilesetProperties: Map<Int, TiledPropertySet> = new Map<Int, TiledPropertySet>();
			var removeTilesets: Bool = false;
            		var firstGid: Int = tileMap.tilesetArray[0].firstGID;
            
			for (tileset in tileMap.tilesetArray)
			{
				//trace(tileset.properties.keys);
				
				if (tileset.properties.keys.exists("remove"))
					removeTilesets = true;
					
				if (!removeTilesets)
				{
					var width = tileset.numCols * tileset.tileWidth;
					var height = tileset.numRows * tileset.tileHeight;
					tilesetTileFrames.push(FlxTileFrames.fromRectangle(Resource.LoadBitmap("assets/data/" + tileset.imageSource, width, height, 0xAA8A5A46, false), tileSize));
					//tilesetTileFrames.push(Resource.LoadBitmapInAtlas("assets/data/" + tileset.imageSource, width, height, 0xAA8A5A46, atlas));
					//trace("tileset.tileProps.length: " + tileset.tileProps.length);
				}
				
				for (i in 0...tileset.tileProps.length)
				{
					var propertySet = tileset.getProperties(i);
					
					if (propertySet != null)
					{
						tilesetProperties[i + tileset.firstGID] = propertySet;
						
						//trace(i);
					}
				}
			}

			var tileSpacing: FlxPoint = FlxPoint.get(0, 0);
			//var tileBorder: FlxPoint = FlxPoint.get(2, 2);
			var tileBorder: FlxPoint = FlxPoint.get(0, 0);
			
			var mergedTileset = FlxTileFrames.combineTileFrames(tilesetTileFrames/*, tileSpacing, tileBorder*/);
			
			var node: FlxNode = atlas.addNode(mergedTileset.parent.bitmap, "combined tiles");
			var combinedTileSize: FlxPoint = new FlxPoint(tileSize.x /*+ tileBorder.x * 2*/, tileSize.y /*+ tileBorder.y * 2*/);
			var combinedTileFramesFromAtlas: FlxTileFrames = node.getTileFrames(combinedTileSize/*, tileSpacing, tileBorder*/);
			
			for (i in 0...tilesetTileFrames.length)
			{
				FlxG.bitmap.remove(tilesetTileFrames[i].parent);
				tilesetTileFrames[i] = null;
			}
			
			FlxG.bitmap.remove(mergedTileset.parent);
			
			mergedTileset = combinedTileFramesFromAtlas;

			
			tileSize.put();
			tileSpacing.put();
			tileBorder.put();

			for (tileset in tileMap.tilesetArray)
			{
				FlxG.bitmap.removeByKey("assets/data/" + tileset.imageSource);
			}

Also you don't need an intermediate sprite, you can override draw() of the sprite you want to draw in repetitive way (in case FlxBackdrop doesn't fit)

FlxGroup can be added to other groups/states, so update() and draw() are called on all the members of the group with exists == true. Be careful, adding to two groups can lead to update() called twice resulting in 2x simulation speed (can be solved by marking the group active = false).

Use arrays when you don't want to update/draw sprites, or they are updated/drawn from other group already.

And also, as you said, use groups when APIs demand FlxGroup, like for object recycling or collision.

I mean compilation surely should work with both VS2013 and VS2015. VS2010 has some problems, like processing assets for an hour, so I don't recommend it.

As I wrote this I also remembered that new VS versions don't install C++ compiler by default and only install C#, so C++ should be explicitly enabled by the user in the installer. (but if this is the case, Android builds should be broken too)

Try Visual Studio 2013 or 2015

Try with older Visual Studio or newer hxcpp
https://github.com/HaxeFoundation/hxcpp/issues/590

It's not slightly worse, it's a lot worse. Don't measure with fps, measure with milliseconds, they tend to accumulate the more code you have, and if they accumulate more than 16 ms per frame, the player will have less than 60 fps. If you have steady 60 fps, but with steady 15 ms (draw + update), you're in trouble.

There is an argument in FlxG.collide() for callback function. You can try reseting the undesired velocity of the box inside the callback.

Also make sure that you didn't mute all the sounds by pressing '0' (zero)

Try asking OpenFL guys at http://community.openfl.org/