Re: 851e SDs
Regarding the 851e SDs, there are 2 slots allocated for SD, namely Duty 1A/5P and Duty 10S. However, the SD for Duty 10S only runs during the evening portion of its duty, because the morning portion of Duty 10S is together with Duty 9P (meaning Duty 10S/9P), which is allocated for DD. Hence, the 2nd SD only runs 1 trip towards New Bridge Road and 1 trip towards Yishun only. The SD for Duty 1A/5P is not of anything special so I will not comment too much about it.
I would also like to reassure you that crossover trips from other services to 851e do not exist entirely, so 851e is purely operated by 851e perm buses only.
Hope this helps. Thank you.