Thru Axle and Quick Release Fork/Frame Compatability?
Simply for your two points:


For a front wheel you can buy or machine a part that will allow you to run a smaller axle on the front than the hub is equipped with. This an adapter to run a 15mm TA hub in 9mm drop outs.




The rear is more difficult as hub sizes get wider with larger diameter thru axles. You couldn't make a 12x142 thru axle hub fit in a 10x135 QR dropout even if you shimmed the space at the end as the chain line would be off.


In terms of frame/ fork design, the design form for through axles offers no backwards compatibility with QR drop out design. I haven't seen and can find no evidence online of an adapter that allows you to run a 9mm hub in 15mm TA drop outs. That being said it could be possible to engineer something that created the required tension to hold the hub in place, but I wouldn't trust a home made solution.