Posted: 16/08/2015 at 17:57
The seedling should certainly be outside all the time. They should be perfectly hardy, but you may want to nurture them in the polytunnel if the weather is hard in the winter.
Acers are pretty promiscuous so they may not look exactly like their parents. But that is the fun bit of gardening.
Stratification is just to kid the seeds they have gone through a period of winter weather. Were they in the 'fridge? I don't know how long is too long for Acers.