You're doing GREAT and unlike a lot of people,,,,you show all your mistakes so others can learn from them!
Forgive me if I ramble,,,,
First off,,,,don't try to cut the backs to match the bezel before soldering them together.The way you did this one is the way to do it or it's next to impossible to try to line everything up when cut to size before soldering.
Trim the excess back off as you did and then you can use your Dremel,files,etc to blend in the back to the bezel but,,,,,,remember that what ever scratches you put in,,,you have to get out so try to use the finest things,files,Dremel tools,emery sticks/paper that will do the job.Sometimes this has to be done in a "series" of the roughest to the finest and then a polishing.
In you photo I cans see where the back isn't quite "blended" in with the bezel yet.
After doing the above,,,try using red tripoli "stick" on a 3/4" or 1" muslin buff in the Dremel.If you've used a fine enough emery,,,,I use a 600 emery,,,,before using the tripoli,,,it will take out any scratches you've made.Then was thoroughly with dish soap and hot water.Dry thoroughly and try using Fabuluster for a final polish.
When "sizing" the bezel wire to the stone,,,,lay the bezel wire next tot he stone to see how high it is,,,a little too high is MUCH better then too low.The bezel wire should only go over the girdle slightly and enough to hold the stone and NO higher.If the bezel wire is a little too high,,go ahead and solder it together.After it's soldered together,,,,take a piece of emery paper and lay it on a very flat stable surface.Rub the bezel wires bottom on the emery going in figure 8 directions and trying to apply even pressure as possible.Check the bezel wire frequently to see if it matches the stone and that it is an even height all the way around.Once you're satisfied with it,,,,,give it a very light sanding with 400 or 600 emery,,,,just to get any sanding burrs off of it.
By now you have more the likely distorted the bezel wire a little so fit it over the stone again.
Some stones,,,,like oval,round,etc are very difficult to tell which is the top and bottoms and in most cases are just a tad different from top to bottom,,,especially if personally cut.Mark the back of these cabs with a Sharpie arrow pointing toward the top so you will put the stone in the setting in the same direction every time you try it and for the final setting.
The bezel wire should fit fairly tight to the stone and before soldering it to the back try to push the stone all the way through the bezel wire WITHOUT distorting the bezel wire.Once you have done this,,,treat the bezel wire as if it was nitro glycerine and place it carefully onto the back.
Told ya I was gunna ramble! LOL
I KNOW that what I've written and all of what others have written may be a bit different and more then likely confusing you as to what to try and do.
There is NO set way to do any of this.You HAVE to weed through everything and try things that work for YOU!
,,,"What should I use,,,one person swears by Zam,,,,another uses Fabuluster,,,yet another says to use red rouge"!Muslim or felt?,,,and the list goes on and on!!!
You're doing very good and I'm SURE that with all the help your getting here,,,you'll do great,,,,,,if we don't confuse you too much! LOL
connrock