Defacto partnership sponsorship – application dates

Hello,
We are currently applying for a two year working visa under defacto grounds.
I'm a New Zelander and my partner is a UK resident.
Under the qualifying income section of my application will be based primarily my partners job offer – thanks to previous industry associates/contacts she has managed to secure an offer of employment for when we arrive in the UK that should in theory fulfil the requirements (under income).
Our major problem is dates. Her new employer is happy to give us flexibility on her start date which means we can spend around 8 weeks doing a little travelling around Europe before arriving in London.
This presents problem when it comes to my application submission dates. We need to send the application more than a month in advance of our departure so that I have my passport in hand when we leave, but this will mean that as far as the immigration is concerned my partners role will kick of nearly 4 months down the track. We're worried that although the offer is legit immigration may question its authenticity and stall the application process, creating a knock-on effect for the whole travel plan.

– Has anyone had experience on if immigration has difficulties around this?
– What are the optimal dates in advance for submitting the application – and how long it takes to process?
– Any other stumbling blocks to watch out for considering we're using an offer of employment rather than on savings to meet the financial criteria?
– Also with both my partner and son have UK passports so it's our eventual goal for me to also gain residency. If for any reason we extend our time in Europe and arrive in the UK at date differing from that specified in the application, will it in any way effect this or future immigration applications?

Another thing to note is that she has taken maternity last year and her income on the books for that period is from a mix of rental income from her London flat and maternity leave income (any issues around that?).
Many thanks
Ben