Mmm, I wouldn't say that templar's lyrium use is so retconned. The fact that Seeker's don't need it is far more suspect to me. With Cullen... it's more that they just don't tell us enough to know why he's not suffering more, since up until now they've suggested quitting typically would have more dire consequences.
Anyway, I find it perfectly plausible that the abilities would still work for a while. I headcanon that they become increasingly less reliable, tho, so that you're lucky if they work or not after a certain amount of time. I also would expect that it would depend on the person, just as the strength of your addiction seems to (i.e. Cullen vs. samson the "chronic" addict).
What I mean is that how Alistair, a Templar that had never taken Lyrium, can use Templar's Ability? That's the point I really don't understand. So:
- Templar can use Templar ability without Lyrium, Lyrium only multiply the power, or
- Templar can't use Templar ability without lyrium
- Lyrium has no effect on Templar's ability & is only a way the Chantry use to control Templars
There's so many interpretation that I don't even get it anymore.
Hmmm I always imagine that Seeker's ability is because they got their connection to the Fade via the Spirit (Cass' Faith Spirit for example). It's very plausible that Templars that once used Lyrium (read: Cullen) can still use their ability for a while, but with diminishing effect over time. I always imagine that Templar's use of lyrium is like forcing a "hole in the wall" to the Fade to keep open with the Lyrium. Whether the hole will seal itself with time, leaving only scars, or will always stay there, Idk.
This can be a good base for a fanfic!
LOL No kidding. It's funny to think of all the questions we had that have now been... answered. I almost miss the uncertainty. 
There's still uncertainty R2s, thousands that I can think of 
- Will Cullen be in any DLC?
- Will there be any Cullen-heavy DLC?
- Will Cullen survive this DLC?
- WIll there be any effect of Keep on Lyrium/Encourage Off Lyrium decision during his Perseverance quest?
- Cullen's spec: Templar or Champion?
- Will Cullen be mentionned in DA4?
- Will we get to see Cullen in DA4?
- WIll Cullen survive long enough to live up to DA4?
- How is Cullen's health later on his life (yes, I got worried...)