If you are not sure about your needle size you can knit a swatch and see how it drapes.
Lace is usually knitted with larger size needles than it will say on the ball band as they will be for ordinary things like socks. The trekking is a sock yarn I think.
Lace is really holey and especially so until you get going so I would knit a little more before deciding to change needle size. But it is your project and there is no right or wrong in knitting, just do what works. If the size you are using isn't looking right for you then change if you want to. It will be fine.
Blocking and synthetic yarn - hmm ? OK here's my 2p on the subject. I have knitted a shawl in an acrylic Mohair, it did open up fine on blocking but there wasn't much stretch in the damp fabric so the transformation pre and post blocking wasn't dramatic. Natural fibres will stretch more - but there are differences between natural fibres too - there's an article on knitty about it I will try and find link for you so you can see.
In the end I think you should use the best quality fibre you can afford at the time for your best projects. A lot of you are doing a test run with cheaper yarns and these will knit up fine and block ok too, they will wash well and will give you an idea of how you like lace knitting. But when you want to try an heirloom project for a dear friend/family memeber that is going to take hours of work and which you expect to be treasured then I would recommend using a 100% merino or a merino cashmere blend. The blocking will transform the finished item, the holes will open up, the project will grow hugely,, and the points will stay nice and pointy, you will be amazed and never want to knit lace in anything else again.
On the other hand you might wish to make a shawl for a new baby and you might want the mum to use it everyday. it will be sicked on and crawled on, as the child gets bigger they will smear it with jam and drag it round the yard. Why would you choose a delicate yarn which has to be hand washed in luke warm water and pinned out to dry everytime it is washed? Well, you wouldn't! Pass me the acrylic. wash in the machine, bung in the tumble drier every day.
You see? There is no definative answer there is just what works. Do what works, use what you have, start knitting tonight, plan a beauty and save up, do what works. and enjoy, it's the journey as well as the destination, enjoy.
Don't fret - it will be fine, it really will. That blocking link http://www.knitty.com/issuewinter02/FEATdiyknitter.html