Apologies for not messaging back sooner. I always prefer to respond when I'm sitting in front of my computer with FCPX and Resolve on it. So, here are some things are important to consider with the FCPX (or even Premiere) workflow for that matter. Both FCPX and Premiere can be the....Wild West (for lack of a better term), when it comes to project organization. Take anything you want from anywhere, and bring it into your project. It can be a dangerous assumption to think that all the media will be contained within that FCPX library (libraries). I think the method that you're going with will be the best for the situation that you're currently in. Assuming he has only one event in his project, all the media should be in that one folder. If not, I'd recommend a new folder hierarchy outside of the FCPX library called High Res Media, or something like that, and then consider going with sub-folders named after his/her events in FCPX. This way, if you need to communicate with him/her, you can say "I'm looking for shot XYZ in even 123......can you take a look at it?", and they will be able to track it down pretty quickly, and it will save you putting all your footage into one "dumping ground". How are you going to deal with the Mac drive(s) on Windows? Mac Drive? It's a great app to do what you're attempting here.
As for a "proper, neat" way to export your footage from Final Cut Pro X, here is my 2 cents worth on how I would do it, if I was the editor. First, NEVER EDIT ON AN INTERNAL DRIVE, ESPECIALLY IN A SITUATION WHERE YOU KNOW A SEPARATE COLORIST WILL BE DOING THE GRADE. lol. An easy way to get all your footage into one new location in FCPX is to, once your edit it done, attach a new media drive to your system, and in FCPX, select your library, open the inspector, hit the "Storage Location" button, and choose a new location for your media - on the new external hard drive. Now, simply hit the Consolidate button in the "Media" section. This will make a copy of all the footage in your project to the new external hard drive. Now that all the footage is there, export an XML, which will be based on the location of the newly moved footage. Once that's done, copy the XML onto that hard drive, and send it to your client. Keep in mind that the media in your FCPX project is now linked to that media on the external hard drive, so it will go offline the instant you remove the drive. FCPX has deleted any of the old media, so once the drive is disconnected, you can simply relink the FCPX project footage back to the footage's existing location. The advantage to working this way is that you'll get an XML based on where the newly consolidated footage lives, however, since you're working with a Mac drive on a PC, the PC will want to assign it a drive letter, and you'll still need to relink in Resolve, but at least you avoid all the hassles of stepping into FCPX Libraries to find your footage.
As far as your export from Resolve to FCPX, you can export with as much handles as you have access to. That's not an issue. When the FCPX editor imports the XML file, it will be pointing to a Windows drive location (I'm pretty sure that's what will happen. I haven't been on MacDrive in a while), so all you'll have to do in FCPX is relink that XML to the timeline you exported from Resolve (which will export as separate clips with as much handles as you think you need), and you should be all set to go. Don't hesitate to drop a line with any questions, as I have FCPX and Resolve, and can help you troubleshoot any issues you might have.