That would be your problem. All FastTESRs are given the same buffer that is already setup properly.
You need to draw at the position, currently you’re drawing near 0, 0, 0
Yes. The vertex format BLOCK requires a lightmap. Lightmap values are from 0-240 with 240 being max brightness. You can use IBlockState#getPackedLightmapCoords to get packed coordinates. You can then use packed >> 16 & 0xFFFF to get the skylight which is stored in the upper bytes and packed & 0xFFFF to get the block light which is stored in the lower bytes.
You might want to look at https://github.com/Cadiboo/Example-Mod/blob/5fe80fde8a41cd571593c02897b06b5822e9a738/src/main/java/io/github/cadiboo/examplemod/client/ClientUtil.java#L240 as it is very well commented. Also note the offset (x, y, z) that is added to the quad in the renderFast method.