Lets take it one at a time.
To qualify for ILR, one of the requirements is that you must score either 75 or 80 points (depending on your category).
You will have to claim some points for past earning where you can select any 12months within 15month period preceeding date of application.
Obviously you will need to submit 2 sets of evidence to justify this claim.
Most people on Tier 1 Geneal that are employed tend to submit payslip and bank statement.
So to answer your first question, i think you will need to submit 12months statement to claim points for past earnings.
With regards to getting letter from previous employer, i am assuming that this letter is to justify your absences. Please correct me if i am wrong.
i think the first thing you have to do is to find out how many days absences you have in any 12 consecutive months counting backwards from the date you intend to submit your application.
If you can do that than i believe some of the senior members will be able to guide you.
Basically the new guidance on absence is that 180days is allowed for any 12months period counting backwards from date of application.
I hope some senior members will comment too so as to guide you.
But i will suggest you go through the FAQ for ILR thread
http://www.immigrationboards.com/viewtopic.php?t=79378
It might also be a good idea to read the SET (O) form and guidance
Good luck.