MultiBlock supports only Cuboid shaped multi-block structures, because it uses BlockRegionComponent underneath.
Theoretically you could make any shaped multi-block, just specifying that the remaining blocks would be "air" or any blocks, however entities with BlockRegionComponent have some built-in behaviours. For example, when you destroy such a multi-block, all blocks in the cuboid regions get destroyed.
None of the three implementations I described fit the shape you have laid out, so you'd have to write your own implementation.
As for your last question I don't know what you mean, the MultiBlockFormItemRecipe has two methods you need to implement, and that's it.
If you provide more details on what you are trying to achieve, and most importantly - why do you want this to be a multi-block, I could try to provide you some more insight or guidance.