Craig Topham has cleared me to contribute to Emacs. I have attached the 
latest version of my patch.

 > All you need to create a new symbol in local context is (let ((my-symbol
 > value)) ...).

I don't think that will work because of lexical binding. For example:

(should
    (equal
     "* A\n* B\n** H1 Capture text\n* C\n"
     (org-test-with-temp-text-in-file "* A\n* B\n* C\n"
       (let* ((file (buffer-file-name))
          (org-capture-templates
           `(("t" "Todo" entry (file+headline ,file 
test-org-capture/entry/headline) "** H1 %?"))))
         (setq test-org-capture/entry/headline "B")
     (org-capture nil "t")
     (insert "Capture text")
     (org-capture-finalize)
         (makunbound 'test-org-capture/entry/headline))
       (buffer-string))))

In that code, if I decide to include `test-org-capture/entry/headline' 
in the `let' then the symbol would be `nil' during `org-capture' even if 
I initialize in the `let' or use `setq'.

I've been also considering testing if the lambda is actually visiting 
the target file when it is called, but that would mean 3 more tests: 
`headline', `olp' and `olp+datetree'. Curious to know if you believe 
that is worthwhile or not.