Okay.......
N from D comes in bottles. It can be tested with barbiturate tests by using a syringe to extract it through the swlf healing lid. This is how it's used in vetenary settings.
You can do the test that you outlined being done in the video however you use an amount of liquid rather than powder. I believe its 1ml but you ahould do a little searching to verify that as I may be wrong. There is definitely an amount of liquid that is suitable for that test though.
With regarss to testing with barbiturate tests and your concerns that it may just contain another barbiturate please take a breath and consider it fully. Whilst what you suggest could technically be true it is illogical for D to do so. First question is... how? D would first need to source the bottle and the aelf healing lids and some way of sealing them. Then D would need to obtain a random barbiturate and mix it into a solution before then bottling it and sealing the bottle hermetically like a lab. Now, for arguments sake lets say that is possible. Now the question is.... why? The procurement of all items needed would need to be both logistically easier and cheaper than obtaining and sending the genuine product. For that to be the case it would need to be done on a large scale as buying in bulk is cheapest. Sure maybe that could be done but it would have to be so large scale that it doesn't really make sense at all and even then the question of 'why? 'still remains. D would still be shipping an ilegal substance across borders and creating a whole ton of extra work for themself when they live in a country where they can obtain the legitimate product for pennies. They' d be risking getting busted procuring the random barbiturate which again is illogical whwn ultimately they could just send a bottlw of any liquid. You might say "well it would test negative for barbs" but regardless of that people would be using the product and failing to ctb. There would be many reports of said failures. Do you ever see them? The only two we've seen involved a lot of treatment and feeling really fucked up. Oh and a positive result for nembutal.
Just because something could happen doesn't mean it will or is even likely to do so.
With regards to testing I'm pretty certain it's legal to send anything for testing once it's booked in with the testing company you choose so sending across borders shouldn't be an issue. It's worth checking details for you specific country but it should be okay. You just sign a document saying you're uncertain what the substance is and that the amount you're sending is all you have and you agree to it being disposed of appropriately by them. There's a service that tests drugs in Ireland (I think) called wedinos that has a document much like that on submission. They don't take submissions from abroad but that's how they handle the legal side of being sent drugs by members of the public.
Hopeful this clears some concerns up. Of course always do your own research to confirm or deny anything as I'm no authority on these matters. Just a (mostly) rational thinking person. I hope.