The taxpayer is responsible for maintaining records of their basis, not the partnership and not the tax return preparer.

Ideally, you get all the K-1s from original investment. Start with capital contribution, add all items of income and gain, subtract all items of loss and deduction, subtract distributions, etc. and consider their share of liabilities to arrive at their basis. 

Capital accounts, even tax basis capital accounts rarely = basis. A negative capital account is possible What do you think you need to correct?

Your beef is with the client with lousy records. You can get some back years 1065s from IRS.

Losses have always been limited to basis. The requirement to attach the basis computation is relatively new.