I'm back here for your expert advice and thank you for your time and help in advance! I also think answers to my questions below will help others or at least I hope so

Background:
So I'm on a Tier 1 General Visa and my wife is a dependent on that visa. Both our visas expire on 24 Aug 2015.
On 7 May 2015 I'm eligible for my ILR and am looking to go for a PEO appointment on or soon after that date. My wife became my dependent after July 2012 so she has a 5 year probation period and will be eligible for ILR in Jan 2018.
Questions:
1- Is it alright for my wife to go for a PBS dependent extension at the same time when I apply for ILR in May? Her current leave ends on 24th Aug so first week of May will be well earlier than 3 months before her visa expiry. Will this be a problem? Will there be an issue for me too considering I have the same visa expiry date? i.e. should we both wait until 3 months before our visas expire?
2 - Is it a given that her visa extension will be for 3 years? Anything short of 2 years 9 months will mean she will need another extension before her ILR and they already cost £2000 so we want this to be the last visa before her ILR. Should I specifically ask for a 3 year extension in a covering letter or something?
3 - When I try to apply online for a PEO appointment and choose myself as a ILR applicant with 1 dependent the total cost comes at £500 lower than if I choose a separate appointment for myself (ILR) and for my wife (PBS dependent). Am I safe to assume that in the first case the system is booking her as an ILR applicant as well?
4 - Finally, if we can apply on the same day, should my application be first or hers? What I mean to ask is, would her application need something (original document) that I will need to submit as well and hence won't have it for her application? If for some reason mine is refused will hers too? Or if my leave is still until Aug 2015(when she applies) will hers not get extended further until after my ILR is granted?
Looking forward to your response!
Many thanks again,
cM