Tier 2 PBS extension

[Mianja]

Hi,
I Entered UK in 25-01-2016 with a dependent visa which is expiring on 03-05-2018
My wife was on Tier 2 sponsored visa and changed to ILR in 2017 based on 5 years continuity.

My question:
1)How soon can I apply for an extension? Is there any limit that I should apply only within 30 days of expiry and not before?
2)How to book for premium service? Is it part of an online application as "Book to apply in person" is only available for Tier 1 or 5 dependent
3)Is "PBS Dependent - Partner" the right online form to apply for? PBS Dependent - Partner
4)what should be my Visa Route and Visa Type?
5)When filling the above form, it asks for Main applicant (my wife) BRP number. Should I be providing the one when she was last on Tier 2 visa?
6)It also asks for whether main applicant work in a shortage occupation? In tier 2 visa her work was on the shortage occupation list but she switched job after getting ILR and it's not the case anymore. I'm confused a bit as it states "If the main applicant has been granted indefinite leave to remain or British Citizenship, you should choose the PBS Tier under which they were granted leave before" while answering for question 5)

Thanks in advance

[Mianja]

Any help on this from moderators are very much appreciated.