This raises a couple of issues. Firstly, as your wife won't have completed her initial 2.5 year stage in the 5 year route to permanent settlement, she will have to apply for an
additional extension to make up the shortfall. The application would be on form FLR(M). It will mean an additional visa fee of £649 (£1049 if applied for in person) and the payment of the new NHS surcharge which will be calculated @ £200 per year of extension. Once all the time spent in the UK can be totalled as 2.5 years she will then have to apply for the next 2.5 year stage to take her to the 5 year residence for ILR.
However secondly, I'm unsure how the Case Worker will view the financial evidence. When applying for FLR(M) under normal circumstances, you would have to submit the last 6 months of pay slips + corresponding bank statements showing that you have earned the equivalent of £18,600 in the 12 month prior to submitting the application. Others may be able to advise on how you are able to comply with this as I assume your earnings will have been outside of the UK.
Will you have employment when you return to the UK meeting the mandatory income level? If so, if you are able to return at least 6 months before your wife's visa expires and have proof of 6 months employment in the UK, I believe your application would be valid.
Edit: Vinny posted link while I was still typing!
