In regards to Carly getting over Andrew Foley, I think it was believable she was over him - because she started getting over him before they even broke up. Basically, he fell off the pedestal she put him on: he had his own problems, he wasn't perfect - which implies Carly's feelings for him were never based on anything real anyway. It was realistic though and true for Carly's character.
As for Carly's "happy ending" with Ben, I don't know why the writers felt they needed to give her one. We can't really know for sure if Sharyn wanted to stay - I personally feel she would've been happy to keep going - but having her leave happily with Ben felt quite rushed and forced. Just the fact that they "fell in love" and got engaged within five minutes is completely ridiculous. Sure, it's true to Carly's character - but that's why she needed to NOT have a happy ending. Things needed to unravel with Ben so she could begin to learn about herself and relationships and the unrealistic expectations she has for them.
On the other hand, the storyline of a young couple learning about compromise and how to live with one another was interesting - but perhaps not for Carly and certainly not with Ben! In some ways, I get the feeling the writers were setting them up to be the new main couple of Summer Bay House. It felt like in killing off Tom they were setting Pippa up to be the single mother and eventual grandmother figure, paving the way for Ben and Carly to take centre stage as the heads of a new family. But then Vanessa Downing and Julian MacMahon quit and I guess they decided to ditch this plan, remarry Pippa and sacrifice Carly...
I think the writers could very have easily written Ben out and kept Carly on - perhaps not while keeping him likeable but why should Ben's likeability be more important than Carly (the longer serving of the two characters)? Ideally, when Ben realised he was AWOL from the army, I would've had him do the "noble" thing (a la Romeo or Josh) and leave Carly without explanation or goodbye, leaving Carly to pick up the pieces, revert to being Carly Morris - and perhaps then (after Ben's departure) realise she's pregnant with his child and see how she feels about this. Of course this would mean she'd be pregnant at the same time as Sophie... Or just have Carly eventually enter another relationship but take it slowly with less unrealistic expectations... I don't know, I just feel like Carly had at least a couple more years mileage as a character.