About 2) and SI:
Let's take the following schema:
HWWW
V
HWWW
V
V
VGGGGGGG
VGGGGGGG
VGGGGGGG
VGGGGGGG
BBBBBBBBBBBBBB
Where
- V: vertically supported blocks, down to bedrock. Vertical stability is infinite.
- W: weighting blocks that require support. SI applies.
- G: ground blocks, usually clay / rock
- B: bedrock
- H: blocks (vertically supported) that support the horizontal weight
Then only the glue of H and W matter. And indeed, the best proof is that very often some of the V blocks will be basic ground blocks like clay or sand.
So if your first W level is high enough from the ground, so that Z/cops/spiders won't hit it, it does not need HP. The 3 first V blocks above the ground however will take the damages from the Z (if they don't have a path to you and go into destroy mode), so these need be stronger.
Also, there is a trick to increase the length of the W bridge you can build. By adding diagonal support, or using multiple faces of the H block, you can increase the supported glue.
HWWW
HWW
HW
V
V
V
B
This allow you to build longer bridges with cobblestone/cement. There is however a max limit independant of the material.
There is a a very good tutorial on youtube by Capoo on SI (
). Most of it should still be pretty accurate. The precise values can be found in the xml, but they do not seem to have changed a lot since a16 (at least for wood/rock/metal blocks).
About 3b)
If you only change H from cement to metal, you will gain 1 block length, but not more, because the first W block will become the limiting factor. If you change the whole level "HWWWW" into metal, then you will reach SI=16 from metal. It might be a good idea to add temporary support during the change
And in the general case, yes you want some margin with SI, because players/zombie/furnitures have a weight, and also in case one pillar or supporting block breaks.
