Onetrail uses de UNSPSC standard for it's classification. An overview can be found here. From this list you have to pick the UNSPSC used codes V.10.

Yes, that's possible. See here the explanation of how to do this.

Yes, that's possible. See here the explanation of how to do this.

Rejected lines are due to errors in your product data file; wrong characters, too many positions, spaces, etc. etc. 
    
When you get rejected lines, you should correct these errors in your product data file and upload your corrected file again.
    
The right data, example notation and additional information can be found in the Product Manager - Product data file guidelines

You get conversion failures because some of your brands, classifications, VAT Codes or UOM Codes still have to be matched. See here the explanation of how to do this.