So I am entering a world of pain?
I'd like to say you have nothing to worry about, but I just don't know.
If you search for "Theobalds Grove to Liverpool Street" on
a ticket site, it will give 2tph on the direct stopper, and 2tph via the West Anglia line, changing at Cheshunt. This should mean that the staff on the Cambridge trains are used to people turning up with Theobalds Grove tickets.
The gateline at Cheshunt will either accept a Theobalds Grove season or reject it. You won't know for sure until you try it.
I would ask a gateline assistant whether he knows if the gate will accept a Theobalds Grove-London ticket. If it does, you'll have nothing to worry about. If he doesn't know, you'll just have to try it out once you have a ticket. If it doesn't work, you'll just have to show your ticket to an assistant every day until they reprogram it.
While you are boarding at an intermediate station on an obvious route between your origin and destination, there's a chance that a gateline assistant may not be familiar with this. I'd probably take a printout of a suggested itinerary between Theobalds Grove and Liverpool Street for the first few days just in case. The ticket office staff should have no trouble in confirming that your ticket is valid, but that would take longer.
There probably aren't more than a dozen gateline staff who work there, so your ticket would stop surprising anyone at all pretty quickly, if it ever does.