So you have modified a brush, and saved it as a preset? That preset should remain available in the preset panel whenever the brush tool is selected. If you then select your new preset, it should definitely have the same settings as when you made the preset.
AFAIK the difference between an ABR file, and a TPL file that consists of brush preset, is where it is located, as controlled by the extension. So if you are happy (this is Windows 7) to enable hidden folders, and go
User\App data\roaming\Adobe\Adobe Photoshop CC\Presets\tools
Find your tpl file and change its extension to abr. Then move it to 'presets\brushes' you are good to go.
That may not be word perfect, but you'll be able to work it out from that.
Does that help at all?
Okay - I tried renaming between .ABR and .TPL but it gives me an error:
"Could not load tool preset [path] because the file is not compatible with this version of Photoshop"
But even if I didn't get it to work, it's really interesting that .abr and .tpl are kind of the same!
I still don't know why my Tool Presets forgets the bitmap brush nibs ... maybe I erased the Brush Presets and this made the Tool Presets lose the nibs?
What I was afraid of was that .tpl files generally didn't remember nibs ... but it seems like Photoshop just somehow corrupted the file. I think I will look at an earlier save of my .tpl