I think that the Darkness plotline worked fine in Season 10 -- in terms of how it began in Season 10, episode 1 and ended with episode 13. Once the Darkness had been rebuffed and rejected by the Vigilante Registration Act repeal, the plotline was effectively over, although it continued to linger awkwardly afterwards.
I've gotten slightly more behind the scenes information on the show since the finale and there were some serious production problems throughout Season 10.
Allison Mack's departure came as a huge shock; the producers knew she was tired out and bored with Chloe Sullivan, but they had expected to get her to stand on her marks and say her lines for one more year -- perhaps by signing her to a 13 episode contract so that she'd have nine episodes off.
Instead, Allison signed for the premiere and then a few weeks of work that would amount to about four episodes in the middle of the season. Production was shocked that, due to the state of their contract, they couldn't even be sure if Allison would be in the finale. In fact, all bets were off for the finale.
Because of Season 10's low budget, they could not book actors well in advance. Fans would expect Allison Mack, Michael Rosenbaum, John Schneider and Annette O'Toole in the finale, but without the money to secure them, all production could do was hope to God these actors hadn't booked other jobs by the time the last filming block arrived.
So they planned what they could -- which is to say they planned the first 13 episodes of Season 10. That's why "Beacon," in which the Darkness is defeated and Lionel is ousted from LuthorCorp, feels like a season finale.
The hope was that by the time episode 13 came, they could figure out what the hell to do for 14 - 22. Maybe Allison Mack's contractual situation could be ironed out. Maybe Rosenbaum would commit to return for one episode.
But by the time they got there, the situation had shown only modest improvement. Allison Mack, John Schneider and Annette O'Toole had agreed to keep their schedules free for the finale. They felt they owed it to the fans.
Michael Rosenbaum, however, continued to refuse to return and at this point, production had devised an alternate plan. Having had different actors play Lex as children and an aged clone, they decided could use Lucas Grabeel, the actor playing the young clone of Lex, to stand in for Rosenbaum. They'd established that clones age fast.
The idea being thrown around in the writers' room: Grabeel, who'd played teenaged Lex in Season 6, would be the restored Lex (original brain, cloned body that would quickly age to adulthood and stop there). Lex would revive the remannts of the Darkness to fight Clark. Clark would defeat both (but Lex would survive). At the end of his final scene, Grabeel would age into Rosenbaum (Lex's face would be lifted from a Season 7 shot and grafted onto a body double's head in a dimly lit shot).
If Rosenbaum changed his mind at the last minute, Grabeel's later scenes could be refilmed with Rosenbaum instead of Lex; the clone would age earlier and faster.
Production planned several episodes with Lucas Grabeel, filmed "Beacon," filmed "Scion," -- but then they lost Grabeel; he signed to play a lead character in SWITCHED AT BIRTH. Production had only been able to hire him as a guest-star; they had no contractual hold on him if he found a regular role on another series. They couldn't even get Grabeel for a cameo in the finale.
It was too late to establish another Lex. Both the preferred plan (Rosenbaum returning) and the backup plan (Grabeel standing in) had failed. And so, the Darkness became the major villain of finale. Rosenbaum agreed to return only in time to film two scenes and some second unit footage.
There is a sad irony in how SMALLVILLE could get Rosenbaum but not Grabeel for the finale.
Cast availability was a constant problem in Season 10. There had been scheduling miracles in Seasons 8 - 9. In Season 10, SMALLVILLE's luck ran out. Kyle Gallner and Lee Thompson Young were unavailable all year, hence the body doubles in "Icarus." Alan Ritchson was able to do "Patriot," but unable to do "Icarus" or "Collateral." The producers reached out to Serinda Swan to reprise her role as Zatanna and the idea of her magic creating Clark's glasses-wearing identity retroactively was thrown around as well, but Swan was a regular on BREAKOUT KINGS. In an interview, Swan expressed her deep regret at being unable to take part in Season 10.
With all this confusion and disarray and uncertainty, many things during Season 10 got lost in the shuffle. The writers, struggling to account for a constantly unclear situation, lost track of whether or not Clark was wearing glasses or if Oliver was a fugitive or if he was avoiding Watchtower or not.
They lost their grip on whether Clark selling the farm was a good thing (it was presented as embracing adulthood) or a bad thing (Martha returned to protest). They failed to come up with a convincing explanation for how Clark's glasses would make his coworkers and friends forget what he looked like without them. They wrote cliffhangers the next episode would ignore.
This situation also made the writers blind to how badly the finale would misfire if they told the story of Clark putting on the costume but avoided any shots of him wearing it -- they were simply dealing with too many impossible circumstances at this point to see straight.
The other massive problem -- the writers worked in Los Angeles but filming was in Vancouver -- so the people scripting the show were often detached from the actual on-set production situation, not realizing that Grabeel could go at any moment or that Allison was totally burnt out. As a result, the team was unable to prepare for impending crisis or take advantage of opportunity.
This is not a SLIDERS in Season 5 situation; the SMALLVILLE team on Season 10 were hard-working and talented and they loved their show. They were simply defeated by budget restrictions, unavailable actors and geography that made it impossible for them to plan, prepare or function.
The takeaway from this, I think, is that networks should never cut budgets so deeply that writers aren't in the same city where the show is being filmed and producers can't lock actors down for filming commitments.
The other takeaway I have is that Season 10's failures don't feel like a really big deal to me because the Season 11 comic books were so good. Sometimes, sticking the landing erases any bad feeling caused by a bumpy flight.