The 'shadow length' problem is difficult to solve. As I noted in the original release notes: "Avoid placing radius-based lights very close to, or even over, shadow caster objects. The algorithm can fail to render a full shadow in this case.". The way the algorithm works is by extending the edges of the objects by a length and filling in the resulting polygon. As you can see from your screenshot the object is very close so the shadow extrudes at a very shallow angle either side of the wall (almost horizontal). This means it doesn't quite end up below the bottom of the screen, then fills across horizontally, not fully filling the shadow area on the screen. I've extended the extrude distance a bit further for the next build, but it's difficult to 100% solve this without extending the polygon for tens or even hundreds of thousands of pixels, which seems like a bad idea (some GPUs might struggle if you send them ridiculously enormous geometry). As the release notes say the best thing to do is just avoid putting lights with a radius that close to the shadow caster, e.g. by putting the light inside an object with an even bigger radius, and only moving the light where the bigger radius is clear of shadow casters.
The multiple lights problem can be solved with a shader (which I've added for the next build). Is what you want is to have 2-3 lights and have shadows only appear where none of the lights can reach them, otherwise having full visibility? If so you can do it (from the next build) like this. For simplicity I've used an example of having 2 lights.
- place 2 lights on the same layer with 50% opacity
- notice that the areas that neither light can reach have 75% opacity (which is what you get when rendering 50% opacity on top twice)
- apply the (new in the next build) Alpha Threshold effect to the layer, and set the threshold to 70
- now you get 0% opacity (transparent, full visibility) where either light can reach, and 100% opacity (opaque, no visibility) where neither light can reach.
With 3 lights, rendering 50% opacity three times results in 87.5% opacity, so a threshold of 80 should work. However the result is, although correct, very strange. I'm not convinced it's what you really want. You get a very unusual pattern of scattered black triangles in odd places, which just so happen to be the intersection of shadow for all three lights. IMO the original example of just using ordinary opacity looks better, but then it doesn't fully obscure areas none of the lights can reach. This again is achievable using additive blend. If you have 2 lights on a layer with 'force own texture' set to yes, opacity 50, and each light using additive blend, then you get a build up to opaqueness instead of just becoming less semitransparent. In other words 50% opacity rendered over 50% opacity with additive blend results in 100% opacity (opaqueness), instead of 75% opacity like you would get with normal blend. IMO this result looks better: instead of weird scattered triangles, you get a clear distinction of full visibility where both lights can reach, 50% opacity where only one light can reach, or fully opaque darkness where neither light can reach. Adding a third light and dropping the opacity to 34% would create a similar three-stage effect and so on. This is already possible in the current build.
In short with creative use of blending, opacity and layers, you can already do some interesting multi-light effects - and with shaders (such as alpha threshold in the next build) you can do even more.