I understand! For what you describe, when using FTP, I log in as user 2. While that may not seem elegant, it "just works", leaves the file permissions alone and precludes me from having to diddle around with a complicated and "messey" process of expanding file privileges beyond those of the owner.
In fact, using the "remapping a subdir" methodology for this on DH, logging in as user 2 is how DH says it should be done.
Note that, while it might not make a difference now, you cannot run CGI from a "remapped" subdir, and at some point this may create a problem for you.
That said, if you do decide to just "control" user 2's files by using user 2's credentials, then there is really no reason to "remap" the subdomain to the subdir, you can just leave it set up as a straight subdomain and use user 2's credentials when you want to manipulate user 2's files.
Another alternative, that I often use and actually like a lot better, is to not provide FTP access at all. I provide all upload and download facilities to these "sub-users", for whom I am responsible, via a "filemanager" type of script that allows me to maintain ownership of all files as my user, but allows them to upload and download within the constraints I have determined are appropriate.
This has numerous advantages from a security and control standpoint, is pretty much the same functionality for the user, and solves some of other "issues" involved with remapping sub-dirs (CGI for instance). It is just a much "cleaner" method of doing this, IMO.
There are lots of these types of scripts around, with varying features and capabilities, and that is how I prefer to accomplish what you are trying to do.
A couple, in particular, that I often recommend are ffileman , and EasyHost (Free version is linked). They take considerably different approaches to the problem and provide different features - both have worked very well for me depending upon what I have wanted to do. Ffileman is more of a "general file manager utility", much like the DH "webftp" application, while EasyHost gives the appearance of being almost a private hosting control panel, which is particularly nice for users enjoying a subdomain on your account.
In either case, all the files they upload end up being owned by you (which as you have seen, solves a lot of problems). In the case of EasyHost, your job of "monitoring" what user 2 does can be made a lot easier with the configurable limits/settings available, allowing you considerable control over what user 2 can upload (file types, disk space used, etc.).
Of course, all this is just my opinion, and YMMV. I've just found that this is what works best for me, and leaves me sleeping more comfortably at night when providing access to my account to another user.