OK, i found a way to redirect the cart_top checkout icon link to guest_step_1:
Go to id_1/template/blocks/cart_top.tpl, change line 29 instance of
href="<?php echo $checkout; ?>"
to
href="/index.php?rt=checkout/guest_step_1"
It's pretty blunt, but it works, and I basically understand why it works.
I don't really understand exactly why the bladerunner approach worked, so I may go back and try my url-direct approach on the 2 checkout buttons on the cart page.
I would still like to hear if these two hacks are a really bad idea from a functional or security standpoint.
I know they're ugly and un-eloquent, but I don't speak php so it's the best i can do without guidance.
Thanks.