Thanks for your kind clarification. You’re of course absolutely right. I was confused because it did not occur to me that WordPress could not write to the local installation (or at least it kept thinking it couldn’t).
It turned out that although http should have the necessary permissions (which I had modified beforehand), the check nonetheless fails, and so WordPress resorts to ftp as the sole installation method. Explicitly specifying in the WordPress config file that direct installation is possible solved the problem, and direct installation succeeded with no problems (cf. eg. http://kb.rolet.com/install-wordpress-plugins-without-ftp-access/).
