As for 'missing identifiers', yep, that's cause the include file is not there.
I can only judge based on the example code that you pasted and, that gives no reason whatsoever for missing identifiers.
No news is good news? I guess if the compiler doesn't tell me it couldn't find it, then it included it.
That is correct. As long as the tax-collector does not come banging on your door, you are good to go

Include File #1: Many of the vars and const, not to mention procedures/functions, that I have used in the past to make coding consistent therefore less errors. It's also simpler and quicker
When you use an editor like lazarus, i would disagree with things being simpler. Include files are much harder to maintain.
If you have build your own library of useful (and reusable) functions it is often much more efficient to put them inside a unit (and make that become part of a (custom) fpc or lazarus package that you can use for every other project).
Of course if those things you mention are heavily tied to a specific unit/form, then i might be inclined to agree with what you wrote but, then i would just put them where they belong: inside the unit (of the form).
Much easier to navigate with a source-browser

What i often do is create a globals.pas (or something similar) unit for a project. Easy to maintain and handle. Of course there might be situations where it is more comfortable to use include files but, since i avoid them, i've learned how to be able to stuff everything into units. Might perhaps be personal preference as well.
I feel that including those types of files is all-around more efficient.
Personally i have such bad experience with include-files with FPC/Lazarus, that i avoid them like the plague and use them as less as is humanly possible.
Other than that, there is not a sane word someone would be able to tell about what is wrong in your situation as there there is simply lacking vital information. In case you are not able to provide that information then try to compile with verbosity set to all, and read what the compiler is complaining about. The compiler output will be very specific
