As far as I know, that 0.1 is yours and only yours. Once they sold it, they gave up all rights to redeem it. Even if they don't touch their other 0.9, you will still be okay. You will be redeeming your 0.1 directly and it won't involve the person that sold it to you.
Yes that is correct. With the way the NXT asset exchange works, once you purchase an asset it is yours and yours only. When it comes time to redeem your asset for your actual NEM, you will return the asset to the issuing account and in return you will get your NEM coins into your NEM wallet. All of this can be done within the NXT wallet so it's pretty easy. Again, once you buy the NEMstake asset it will show up in your NXT wallet as YOUR asset so there is no risk of anyone not giving up their stake when it comes time to redeem.
Thank you for your help. This is what sets NEM apart, very kind of you all to answer. Just bought another 0.1 haha...