Your first patch should definitely not be saving out as a dot! are you using the export all? If you go into your mari projects tab you can archive the scene and send it to me, i'll then be able to tell you why

If you have made a mistake you will need to mask it out. However the mask can then be baked onto the layer. When it comes to mari and masking there is a bit of a learning curve. You should get into the habit of breaking everything up into separate layers that way a mistake won't cost you the entire texture.

Also don't worry about the small mistakes for now, you'd be amazed at what you can do with render passes and post production. Getting the image out of Maya and into Nuke or other composing app can let you get a grasp on what needs fixing and what does not. It will also make a huge difference to how your image looks.

If you want to jump ahead a bit and see what's possible in Nuke (you can now get nuke studio free) I'll be happy to send you my nuke file and final render so you can have a look.

I tried to cover a full pipeline in this tutorial because IMO if you don't understand the full process of what goes into to a final image you'll never know where the work needs to be put in to make an image better. You'll then end up endlessly rendering images in Maya trying to get a look that would have been much more achievable in another part of the pipeline.

Everything you are trying to do right now is difficult, stick with it and try not to throw the PC out the window i've come close more than once myself.

All the best
Dave