php can write to tmp dir, but not wp for plugin update?

Sorry – I’m probably being thick. Spent hours looking at this, read many answers to similar issues, but rusty on php and I can’t understand yet what’s going wrong. Might be mis-interpreting what I’m seeing, making big assumptions / false-premise etc. OR: maybe what I want to do can’t be done?

WP 3.92 running on my PC, Windows 8.1, IIS 8.5, PHP 5.5.15

Essentially: plug-in updates:
“Download failed. Destination directory for file streaming does not exist or is not writeable.”

I’ve tried many experiments with ACL settings, php settings and windows temp directory settings etc. but simply here: testing:
F:\TEST ACL’s set to everyone full control, plus to be absolutely certain, the php apppool set to full control
ALSO: allowed same privileges for the entire WP directory and sub-directories (for testing).

If I do in wp_config:
define(‘WP_TEMP_DIR’, ‘F:\TEST/’); then I still get the error.

If I do in wp_config:
define(‘WP_TEMP_DIR’, ABSPATH . ‘wp-content/’); then it works

I wanted to use a directory outside of the WP path. Is it simply not doable?

To check if the issue seemed to be with PHP, I inserted the below in header.php in the responsive theme:

<?php echo 'get_temp_dir() = ' . get_temp_dir() . '    ';
            $filename='F:\TEST\test.txt';
            If(!file_exists($filename)){
                echo "file_exists function could not find " . 'F:\TEST\test.txt';
            }
            else{
                echo "file_exists function FOUND " . 'F:\TEST\test.txt    ';
            }

            echo 'is_writable(F:\TEST\test.txt) = ... ' . is_writable('F:\TEST\test.txt');
            $myfile = fopen('F:\TEST\test.txt', "w") or die("Unable to open file!");
            $txt = "John Doe\n";
            fwrite($myfile, $txt);
            $txt = "Jane Doe\n";
            fwrite($myfile, $txt);
            fclose($myfile);
            ?>

In my header I got:
1st run: get_temp_dir() = F:\TEST/ file_exists function could not find F:\TEST\test.txtis_writable(F:\TEST\test.txt) = …

Refresh: get_temp_dir() = F:\TEST/ file_exists function FOUND F:\TEST\test.txt is_writable(F:\TEST\test.txt) = … 1

And I opened F:\TEST\test.txt in Windows Explorer and the writes had been made.

My interpretation: php can write to the directory I specify. WP_TEMP_DIR seems to tell WP the folder to use for download/unpacking etc. and is happy with the path form if the folder is wp_content or a subdirectory but not if something else. So – my interpretation … WP is doing something different? I don’t understand.

nb: not using open base dir or doc root stuff. I do use fgi impersonate … & normally basic authentication (windows user though planning on using LDAP eventually) (and set relevant ACL too) – but tried it without & anonymous … same thing.

Is there a quick answer? Should it work? I have no professional experience in this stuff … my day job is just a grade 4 local authority administrator dealing with market traders … this is just a means to an end. I wasn’t expecting this kind of problem. Just a pointer would be very welcome! Or, sigh, do I have to go through all the code line by line to understand it? 🙁 I have many competing priorities from other complex non-php projects / demands on my time and didn’t want to have to do that just for this little problem. 🙁 I’m very shy (diagnosed autism) and asking is actually very difficult for me … I feel very embarrassed and awkward. It’s not a last resort … but getting there. Ta. (Hoping it’s dead-simple and me being thick!).