How does a DNG-file work? Is it“100% safe”?

[Hans]

I have scanned slides in Vuescan and got a Vuescan “DNG TIFF-file” (a sort of dng-file) of approx 35 MB (2000x3000 pixels/48 bit). I have adjusted the imagers in Lightroom 3.6 and then used “Update DNG preview & metadata”. The file size decreased to 23 MB, i.e. by 12 MB or 1/3 of the original file size!
(Normally I have not used “Update DNG preview & metadata”, but I saw an advantage of seeing the Lightroom adjustments in my MS Expression Media 2.)

Now, in Lightroom the file is still said to be 2000x3000 pixels, but in MS Explorer it says approx 700x1024. I assume that this latter might be the size of a new revised Jpg-preview… Then I used “Reset” (and “Update DNG preview & metadata”) in Lightroom and the file size in MS Explorer increased to 2000x3000 pixels but the file size in MB is unchanged!

I thought a dng-file (even if they are made through Vuescan, which is somewhat different compared to dng-files made from “proprietary raw”-files) was impossible to change, so my questions are:

1.   Have I lost all original image information from my scan?
2.   Is it true that the image information always is safe in my dng-files and that I only can change the “instructions”
      (Parametric Image Edits, PIE) and perhaps the Jpeg preview?
3.   Can I now reset completely and get back to my original file (of size 35 MB)? Or, perhaps it is of no value to reset completely?
4.   Can anybody explain what I may have done?
5.   Where can I get more information on how the dng-files really work? Perhaps there are suitable threads on the internet that 
      you can recommend?

After having spent hours of searching on the internet and in the literature, I appreciate any information on what I might have done and what to do next!
Thanks in advance!

[Hans]

I have learnt more about Vuescan DNG TIFF-files since I wrote the above.
I noticed earlier that "Update DNG previews & metadata" in LR3/4 changes the file.
I have now found that by opening Vuescan DNG TIFF-files in my Photoshop CS3 (i.e. first automatically opening in ACR and then in CS3) and then just looking at the images, closing them down (without any saving!) ALSO CHANGE the Vuescan DNG-TIFF image!

After a contact with Ed Hamrick at Vuescan, he found out that there has been a JPG ENCODING of my submitted Vuescan DNG TIFF-files!
There has been a compression in LR and PS respectively and instead of 48B colour there is now only 24B.
So the answer to my own question above, "Is Vuescan DNG TIFF safe to use?", is:

      NO, VUESCAN DNG TIFF IS NOT SAFE TO USE!

(I thought a dng file would never be possible to change! But I was wrong!)

Nowadays, I apply "write protection" on all my Vuescan DNG TIFF-files, which makes it possible to view them in PS. And all adjustments, if any, of the images are stored then in an xmp-file.
Furthermore, I do NOT apply "Update DNG preview & metadata" in LR any more! And the "write protection" mentioned will stop me as well.