I can definitely see the appeal in having something like a help desk staffed with tech support people. It’s always nice to have someone there that can take the time to talk you through a process, and help you to attempt different investigative methods and fixes for your issue.
I don’t know if 343i would be able to do this, as I don’t know how many people they’d need available for it to be effective. From what I can gather based on the types of updates and support Halo 4 has received, 343i doesn’t have any staff members to spare. If they have the budget for it and can get approval for it, then it might be possible for them to hire a different company to provide a dedicated help desk, but I’m not sure that providing such a thing is common practice in the gaming industry. (Why a different company? It’s common for one company to use a second company to provide tech support: it prevents the first company from having to reinvent the wheel, and it allows them to benefit from the second company’s experience.)
So I don’t know the precise reasons why 343i doesn’t offer a dedicated help desk, but I hope the above paragraph can at least provide a small amount of insight.
What I do know is that they have provided a few channels for submitting information on bugs, as well as a few sources of information that can help users gain insight into their problems. I’ve documented these in the spoiler below (based on the categories you listed: bugs/exploits, frame-rate issues, and networking).
Bear in mind that these are somewhat general answers; I’m sorry if you’ve already heard it all before and it’s not helpful.
Bugs and exploits
There are different places to report bugs depending on what sort of bug you’re experiencing.
If it’s something specific to a Matchmaking playlist, map variant, or game variant, then you should post in the Matchmaking Bug Reporting thread under War Games Feedback. To name a few recent examples, things like underwater spawn points and Ordnance drops falling off the map would go in that thread.
For other bugs, like unreadable discs or inaccessible DLC, you should try the Halo 4 Known Issues thread. I know it doesn’t get a lot of public 343i responses, but to the best of my knowledge, they do read it.
Bugs with gametype scripting could technically be crossposted to the community’s long-running Custom Games thread, as bs angel has confirmed in the past that 343i staff read it regularly. However, you should not rely on that as an official bug reporting channel. Basically, for Custom Game bugs, post in one of the official threads, and if you want to go the extra mile, then you can also post in the Custom Games thread.
Finally, system exploits (like network manipulation) should be reported to Xbox LIVE Enforcement, by Filing a Complaint using the Xbox 360 menus. These complaints are reviewed within 24 hours of being filed, if I recall correctly.
Frame-rate issues
This is a trickier topic because frame rate issues stem from the game overtaxing the Xbox 360’s hardware. Sometimes, this is avoidable; sometimes, it’s not.
One good rule of thumb: if you’re playing split-screen (and not on Forge Island), you’re gonna have a bad time. This is true for Forged maps in Reach and for most maps (Forged or ordinary) in Halo 4. :\
That said, on Forged maps, there sometimes are some measures that mapmakers can take to alleviate frame rate issues on both single- and split-screen. If you notice frame rate issues on Forged maps, then consider posting about it in War Games Feedback. Sometimes, frame rate lag only occurs at certain camera positions and angles, so be sure to mention where on the map you get issues, and where you’re looking (give this information for all players if on split-screen).
I’m a little iffy about posting frame rate issues in the Bug thread, as it’s less a bug and more an difficult-to-avoid consequence of the graphics engine trying to do X things on Y processing power. I personally would take whatever playlist contains the problematic map, and post about the issue in that playlist’s thread.
It may also be worth looking for the map on other Forge websites and commenting there, in the hopes that the original Forger will see your feedback, make adjustments, and send an updated version to the Community Cartographers. (From what I’ve seen and heard, maps don’t get used if they can’t be found. If it’s in Matchmaking, then I almost guarantee that it has a map thread on one of the Forge communities.)
Networking issues
To my knowledge, there are two resources that could help to shed light on networking issues in Matchmaking. One of our community members has created a networking thread that includes a tutorial on opening one’s NAT, and 343i has provided an overview of the Matchmaking process here. The latter source provides a few details on connection-based matching (e.x. the use of latency estimates to find potentially good sessions, before the servers query for actual latency values).