Here is summary of my answers to your questions:
Your husband is eligible for reinstatement and there is no time limit on his reinstatement eligibility due to his "veterans' preference". A reinstatement eligibility allows your husband for followings:
i- Reenter the Federal competitive service workforce without competing with the public in a civil service examination.
ii-He may apply for any open civil service examination.
iii- He can apply for Federal jobs open only to status candidates.
Under reinstatement eligibility, your husband can only apply for a job which is equal to the grad that he last held or to the lower grad that he last help. However, your husband wants to apply for higher grand than he last held then he will have to compete through open job. But since your husband is applying for the same position that he last held then he does not have to compete through open job announcement and he is eligible to reinstatement to that position, which was open to the status candidates.
Your husband is eligible to apply for internal job opening under merit promotion program, which is used by the agencies to fill internal position and it includes individuals with reinstatement eligibility.
USPS is a federal agency and they are bound to consider candidate with reinstatement eligibility for their internal job opening through their merit promotion program. A reinstatement eligibility does not automatically guaranty a job to a reinstatement eligible candidate but it just allow such candidate to apply for that job and gives right to be considered by the hiring agency. It's sole discretion of hiring agency that whom they want to offer the job based upon the skills and qualification needed for such job. Therefore, it's your husband's right to be considered for the same job that he held before with USPS, but selection is up to USPS.
Please click on these links for details: http://federaljobs.net/fed.htm . http://www.usajobs.gov/EI2.asp
As you stated that you prefer second opinion, I am going to opt out of this question, so some other expert could look into the matter and help you further. I did my best to provide you the best answer and I wish you and your husband all the best.