I believe this would be considered reasonably reliable. I would try to implement a procedure to obtain these documents from a third party (other than the applicant/borrower) when possible. See page 15 of the CFPB’s small entity compliance guide: https://files.consumerfinance.gov/f/201308_cfpb_atr-qm-implementation-guide_final.pdf .
As for the social security income, IMO you would need the letter to determine whether the benefit is set to expire in three years. See excerpt from Appendix Q below.
Regulation Z, Appendix Q, B-11. Social Security Income. Social Security income must be verified by a Social Security Administration benefit verification letter (sometimes called a “proof of income letter,” “budget letter,” “benefits letter,” or “proof of award letter”). If any benefits expire within the first full three years of the loan, the income source may not be used in qualifying.
Notes:
i. If the Social Security Administration benefit verification letter does not indicate a defined expiration date within three years of loan origination, the creditor shall consider the income effective and likely to continue. Pending or current re-evaluation of medical eligibility for benefit payments is not considered an indication that the benefit payments are not likely to continue.
ii. Some portion of Social Security income may be “grossed up” if deemed nontaxable by the IRS.