From function: directing error from external command to window and open files in specific windows of current tab

I need help with the following. I've googled for half an hour
but don't seem to be able to formulate the questions correctly.
As usual it's probably in the help but I don't know where to look!

* How to call an external script from inside a function without
affecting anything inside Vim, but getting its error message
if any, preferably in a window without polluting any important
existing window.

* How to open a specific file in a specific window of the
current tab from inside a function, given the path to that
file as a string argument. There are three windows like this,
and I wan't to open two other files in a and c, so I guess
I'll also need to know how to identify those windows, with a
on the left as well as the mirror image with a on the right:

+--+--+
| |b |
|a +--+
| |c |
+--+--+

/bpj

--
--
You received this message from the "vim_use" maillist.
Do not top-post! Type your reply below the text you are replying to.
For more information, visit http://www.vim.org/maillist.php

---
You received this message because you are subscribed to the Google Groups "vim_use" group.
To unsubscribe from this group and stop receiving emails from it, send an email to vim_use+unsubscribe@googlegroups.com.
For more options, visit https://groups.google.com/groups/opt_out.