Just a quick update:
Of course BTC can break to the downside, too. The trendline might represent strong support, but as you can see from the charts the price might hover around the trendline for a while, spiking in both directions. My shorting strategy would be to wait until BTC firmly breaks the trendline to the downside and comes back to retest it. The volume at this point will show how probable it is for BTC to get rejected/fail the retest and then dive downwards. I would never enter a short position while BTC holds this line.
For now I'm long from 3670.
Going to update this thread with more charts soon.