Growing climbers through shrubs

No to the first question !
It won't strangle it, i do what you're thinking of doing and it works very well.

Secondly, you would trim/cut the hedge when the Clematis had been cut back down.
If you've cut back your Clematis in the late winter/early spring (as a lot require) cut the hedge then, if you cut the Clematis after it's started shooting you may lose that years flowers.

Your only option is to make sure you choose a prune group 3 clematis, one of the later flowering jackmanni hybrids. These need cutting down by mid February to about 8 inches from the ground, so will give you a chance, in April/May, to cut back the Pittosporum. You will not, though, be able to cut it back again later in the year without going very, very carefully round the clematis.

Indeed.
Topiary would be unsuitable !
(sorry, that's my silly side showing!)

Be careful to match the vigour of your climber to its host. For example, clematis montana is a thug and will kill anything that is not big, well established and able to look after itself. Something like C.cartmanii 'Joe' may be more suitable for a five foot high shrub

Thank you! That's all really useful. I'll take a look at that those Bamboo and Andrew. Love your photos Louise!

Thank you Elo :))