Overlap between FLE and FLR(M)

My wife and I attempting to a apply for Mortgage and to renew her spousal visa so she has the longest time remaining on her visa at the time we apply for the mortgage. We need at least 2 years 6 months for the agreement in principle to be valid. Her original spousal visa was valid for 33 months and expires at the end of May, and she arrived in the UK on 9th of September 2014. We understand that she can apply for FLR(M) from 28 days before her 30 month 'anniversary' which is the 9th February.
Our query is will they carry over any remaining leave from her spousal visa to her new FLR(M) giving us 2 years 8 months on the new visa?

Thank you in advance for any help you can offer.

There's no such thing as an FLE(M) visa - I'm assuming you mean her initial 33-month spousal visa?

Yes, you are right - she can apply for FLR(M) 28 days before she reaches 30 months in the UK. If she entered the UK on 9th September 2014, she can apply for FLR(M) from 9th March 2017.

However, FLR(M) is only issued for 2 years and 6 months (30 months) exactly - no more, and no less. So, she will have 2 years and 6 months on it, regardless of when it is issued.

It's only the initial spousal visa that is issued for 33 months, which was done to give a her 3-month leeway to enter the UK and still qualify for ILR after 5 years.