extending fiance visa?

I've had a look into my girlfriend getting a fiance visa, and suggested she do this, but I see it is only valid for 6 months.  I love her, but I think I'd be more comfortable with a longer engagement than this.  Is it possible for it to be extended, or for her to get the visa dated to start 6 months in the future, and then be over here as a normal visitor for 6 months before this?

Also, her current passport,when under scrutiny will reveal some overstays.  She was never stopped leaving the country, but the stamps she has look suspicious.  She's due to renew her passport soon anyway, but would it be better for her to apply for the fiance visa with the new clean stamp-free passport?  We've got plenty of evidence of being together anyway (including a police report of a car crash we were in a year ago!)

you can only extend a fiance visa under extraordinary circumstances.  Simply wanting a longer engagement is not a good enough reason, sorry.

I don't think it really matters about the passport, to be honest. The fiance visa application asks about travel history and any previous passports you have so you're not going to be able to hide the overstays.  But if she's due to renew her passport soon it might make sense to get it over with before you start the visa process so that she won't run into the problem of needing a visa put into a passport that's valid for less than 6 months.

I don't want to give you any misinformation, so I won't answer your questions, but I will say that when I applied for my fiance visa I included both my old passport and my current one.  I think they ask you to do this, but I'm not 100 percent sure.  

I do know that they on the fiance visa application they ask you to list every visit to the UK and the dates you were there (arrival and departure dates.)  You shouldn't lie, it's better to reveal all because lying may screw up your chances of her being allowed back into the UK again because now there are strict rules about deception.

cross-posted with Springhaze.

You can have as long an engagement as you want.  You just can't necessarily be together for the entire engagement.  Simply wanting a longer engagement wouldn't qualify you for an extension. 

You may want to look at this thread:

http://talk.uk-yankee.com/index.php?topic=43463.0

If your gf fits the qualifications, this might give you the time you feel you need, but I also think that if you then decide to get married during the time she is in the UK, she would need to go back to the US to get a fiance visa (or you could both go to the US, get married, she could then apply for a spousal visa and re-enter the UK that way).