re: Why not build a massive containment dome...

One reason hydrates get formed and gunk things up is because the seawater gets in with the oil. That's why the current cap (which doesn't have a tight seal) deliberately leaks some of the oil out, to keep out the seawater.

Since you need a (reasonably) tight seal, having a cavernous metal shell doesn't really do anything for you if you're pumping the oil out as fast as it comes in.