Urgent query about my VISA extension & applying for ILR

Dear Macchi,

I have an urgent query about my VISA extension & applying for ILR.

Currently, I am based out of UK on Work Permit ICT Tier 2, since May'10.
My VISA is valid from 17/05/2010 (as mentioned in the VISA stamp) & I arrived in the UK on 24/05/2010. I am not sure about the actual application date, which was done from the company side
Since then I have got my visa extended twice & my current extension is expiring on 31/05/2015, which would be almost 5 yrs in the UK

I have got the following queries -

1. Can I apply for ILR after May'15
2. Can I apply for VISA extension after May'15 staying in the UK
3. Can I apply for VISA extension/or a new VISA immediately after going back
4. What are the other option to stay back & work in the UK

Many thanks