In this case your Asus P8P67 has been released with PCH controller Hub, as you can check on the following chart RAID 10 should be supported by it, however several manufacturers have customized Intel devices with some specific requirements into their systems
Regarding your second question, with RAID 10 the PCH controller Hub can use up to 4 HDD for I/O as well.
Therefore in order to know why these features have been modified I recommend you contacting the motherboard manufacturer.
You can confirm this information at the following url:
Your understanding of RAID needs to be rewritten.
RAID 10 is RAID 10 you are using RAID 10 Intel dose not support RAID 0 + 1 nor can it be made to do so other then by Intel.
In a 4 disk RAID 10 setup (6 disk RAID 10 (3 sets of RAID 1 in RAID 0) could be done but Intel don't want to support it) you will only get double READ speed of a single disk as two sets of disks are in RAID 1 which are then RAID 0 together. Since data on a RAID 1 set is the same with two RAID 1 sets in RAID 0 you can only get double READ speed of a single disk.
If you want more read speed go RAID 5.