What application is using a particular file

I think Apple wants the people that apply to know how the Xcode tool chain works and have invested enough time to be able to submit a real app for review rather than being people that can fill out a web form to apply for a ticket.

Now, since you've found us and asked for help with lots of detail, I think you are very close to qualifying for a chance to win a ticket, but you might need to ask a second question staking out one specific step that you don't quite get.

That will entice someone who is an expert to help you over the one item you have identified as the single biggest obstacle. At that point, you are free to ask again with your next biggest obstacle, but as you have surmised, asking too much of an answerer by submitting a "wall of question" is generally frowned upon on Q&A sites. Your nice pictures do help ameliorate the large amount of text, but it still seems like a three part question (or more) to me as written.

Lots of detail does not imply lots of thinking. We can't do your thinking for you, but we can attempt to help if you've shown clearly what you think and what specifically you need help with.

I think I've got an answer.

Googling around I've found references to "grayed out" problems and mentions of the file's creator attribute.

So a quick Google for "osx file creator mark" pointed me to SetFileand its sibling GetFileInfo.

Running a quick GetFileInfo on the files I get:

Mac:samples jjarava$ for i in 201507*.TXT; do getfileinfo "$i"; echo .; done
file: "/path/to/samples/20150728 - Source Fragment 3 (BAD).TXT"
type: "????"
creator: "????"
attributes: avbstclinmedz
created: 08/03/2015 13:58:18
modified: 08/03/2015 13:58:18
.
file: "/path/to/samples/20150728 - Source Framgent 1 (Good).TXT"
type: "TEXT"
creator: "\0\0\0\0"
attributes: avbstclinmedz
created: 08/03/2015 13:58:18
modified: 08/03/2015 13:58:18
.
file: "/path/to/samples/20150728 - Source Framgent 2 (Good).TXT"
type: "TEXT"
creator: "\0\0\0\0"
attributes: avbstclinmedz
created: 08/03/2015 13:58:18
modified: 08/03/2015 13:58:18
.
file: "/path/to/samples/20150728 - Source (BAD).TXT"
type: "????"
creator: "????"
attributes: avbstclinmedz
created: 08/03/2015 13:58:18
modified: 08/03/2015 13:58:18
.
file: "/path/to/samples/20150728 - Source Fragment 3 copy (BAD).TXT"
type: "????"
creator: "????"
attributes: avbstclinmedz
created: 08/03/2015 13:58:18
modified: 08/03/2015 13:58:18
.

All the "working" files are of type: "TEXT", and all the "not working" ones seem not to have a "type" defined...

Actually, running the following to change the file type:

Mac:samples jjarava$ setfile -t TEXT "20150728 - Source Fragment 3 copy (BAD).TXT"
Mac:samples jjarava$ getfileinfo "20150728 - Source Fragment 3 copy (BAD).TXT"
file: "/path/to/samples/20150728 - Source Fragment 3 copy (BAD).TXT"
type: "TEXT"
creator: "????"
attributes: avbstclinmedz
created: 08/03/2015 13:58:18
modified: 08/03/2015 13:58:18

And that file can now be selected in the "Open" dialog in Excel!!

The question is where does the "type" field come from, and why it's set in some files and not in others, but at least there's some "logic" to the issue!!