There are, some are SAS2 and PCIe 2.0, some SAS2 and PCIe 3.0, some SAS3 and PCIe 3.0, for Unraid use with spinners (without expanders) they will all perform similar.
Yes, there's no limit with any sizes for the foreseeable future.
Some are IT mode only, e.g. 9207, 9300, but flashing to IT is easy if needed.
Whatever works best for you, in terms of total cost and number of available PCIe slots.
Yes, cables need to be forward breakout like that one, reverse breakout look the same but won't work for this.