conclusion: use TORA as the LDA unless tere is displaced threshold.
This is incorrect. The LDA may be shorter than the TORA even without a displace threshhold if a portion of the runway needs to be used to satsify the runway safety area requirement. See, for example runway 09L at KATL (
http://aeronav.faa.gov/pdfs/se_158_20SEP2012.pdf) where the TORA is 12390 and the ASDA is 11730.