However, the main issue is that when you can quote multiple comments (when you are in a firehose view) you tend to send a single post for multiple replies; I have no idea how that could be shown in the tree view.
Essentially a firehose is a superset of a tree structure, if your unit is a post.
One problem is that when people are manually quoting a post, they can edit that quote down to only include the relevant portion of the post they're replying to, and depending on the forum culture unnecessary full-quoting is actively being discouraged.
Automatically added quotes on the other hand don't have such a luxury, so you'd get a big bunch of full quotes, which wouldn't really be ideal, either.
The potential for problems would arise when everyone Replied using a root-level post to the firehose. This could be mitigated with prominent Reply links on every post in the firehose, with the root-level contribution form deemphasized. So you can quote multiple people but you still chose a parent by choosing which Reply link to click.