I have purchased a dozen Air China flights through Lifemiles (all biz class). Never had an issue with cancellation (touch wood!). Seat selection can only be done on line if you purchase your ticket from Air China on line. For LM issued tickets you just call up. Very quick and easy.
When using top up points plus pay, use the slider to carefully determine where the 'sweet spot' is. It's roughly about 42% of the total miles needed for an award. The slider will - when on the far right - jump from USD15 per 1000 mile increments to USD20-30 per 1000 mile increments. That's when you need to click back one level to ensure you're getting the best value. (Not at the far left of the sliding scale the top up points will be more than USD15 per 1000, but somewhere in the middle the incremental price actually goes into the negative.)
The 145% bonus only applies to outright 'buy miles' through the home page of your LM account, and not to top up points during a booking. Still, the difference between outright buy miles at .01375 and topping up at .15 is not huge. So you can minimize any risks by only holding the 42% required, and topping up at time of ticket purchase.