mkt,
good questions 🙂
1. downlink DTX off means that all TS that are not sending data/voice/signalling will not transmit anything (0 power).
however there is an exception : the BCCH TRX (all the 8 TS) will always transmit a signal, even if there is nothing to transmit.
Since the C1/C2 are calculated on the BCCH frequency, then having or not DTX will not change a single thing.
2. DL power control and DL DTX are a way to decrease downlink interference in a network. Combining them is the most effective, but just using one of them is possible.