I like those, that is a good pick.
As to your hoses being 5 feet off the floor, the inspector does not care about that. What they want is a backflow period. As a former Auditor it would not matter to me that your hoses are 5 feet off the floor - all I would go by would be the requirement of the audit or the standard requirement and if it said you needed backflows... well, you need backflows.
As to certification - is that a state, county or city requirement for your area?
If it is, then you will be required to show proof of certification - such as in the Cook County, IL (Chicago) where a plumbing company does the checks and issues a certificate for each one.
If not a requirement, most companies we deal with simply document in writing that the devices have been checked. Most would place on there PM program, check them, document the checks and that is proof for an Auditor they were in fact checked.