Tier 2 Dependents application

I am currently working in the UK on a Tier 2 visa with an A-Rated sponsor. However, my Certificate of Sponsorship has expired prior to applying for my wife and daughter.

HR is willing to issue a general letter stating my salary, period of employment and my current address as a document in support of my dependents, but do not know how to / unwilling to issue a written undertaking in support of my family.

1. I would like to know whether the above (limited) letter is acceptable for Visa application purposes.

2. If not, can a combined bank statement from (2 seperate) bank accounts be acceptable?

3. And how much would we be aiming to show in total?
3a. £2835 in available funds (£630 x 3 = £1890 + £945)
https://www.gov.uk/government/uploads/s ... _11-15.pdf
3b. £1260 in available funds (as we are only applying for 2 dependents)

Thanks!