With regards to your 1st query, i had the same question and asked PBS for advice and this is what i received. which simply states you need to apply before the COS expiry date, hope this helps.
Dear
Thank you for your email.
In response to your enquiry, a certificate of sponsorship issued by a sponsor under Tier 2 and Tier 5 is valid for three months from the date of issue.
The first dates that you see on the first COS details page are the date the COS was assigned and the date it expires. By expires it means the date by which the migrant must have used the COS to obtain either Entry Clearance or Further Leave to Remain.
On the second page of the COS details you will find another set of dates; "Work Dates", these are the dates that the sponsor has requested the COS for, and can be anything up to and including 3yrs.
If the migrant does not use the certificate of sponsorship within this period to gain entry clearance or leave to remain it will automatically become invalid.
During this period the migrant cannot be issued with another certificate of sponsorship by any other sponsor.
If a migrant does not want to take up the employment or join the course because he wishes to take up an offer of a job or course from a different sponsor, he must contact the original sponsor to cancel/withdraw the certificate of sponsorship.
Please note the applicant should apply no more than 3 months before the start date on the COS.
I hope that this is useful.
Yours sincerely,
Naurin Fatima.
Customer Service Advisor
Customer Contact Centre
North East, Yorkshire and the Humber Region
UK Border Agency
PO Box 3468
Sheffield
S3 8WA
Telephone Enquiries:
For sponsors requiring information on sponsorship under the points-based system:
0300 123 4699
For Bulgarian and Romanian applications:
Telephone: 0114 207 4074
Fax: 0114 207 5861
www.ukba.homeoffice.gov.uk
________________________________________
From: Sent: 10 October 2011 12:33
To: Sponsorship PBS enquiries
Subject: COS Allocation and application made
Dear Sirs,
Our employee has recently made an application for the Tier 2 (General) visa on the 7th of October 2011 . He is currently on Post study work (Tier 1) which expires on the 23rd of October 2011. We had issued him with a certificate of sponsorship on the 20th September 2011 and the starting date for his COS as the 1st of October 2011.
As per your tier 2 guidance policy it states that the applicant must apply within 3 months from the date the COS was issued and should apply no more than 3 months before the start date on the COS. As he has applied for the tier 2 visa on the 7th of October and the COS start date if 1st od October we would like to know if his application is still valid.
Kindest Regards,