Your calcs seem right for the 28AH (ie, 28AH@C20 = 1.4A for 20 hours etc), and your BS reference reeks of wisdom**.
I'd charge the unit FIRST and then test.
Reason - thru storage they are usually run down; most appliances state "charge before use" to blow away the cobwebs (sulfate etc) and prevent further deterioration. (FYI- battery capacity usually increases after a few cycles.)
Besides, then you start from a known condition - ie, fully charged.
Re other specs, I am going from the spec sheet I downloaded, and some of the info is ambiguous - ie, a 2.5A dc-plug charging limit.
But that's what their techs should be able to clarify - including charge and use simultaneously (why not? - after all it's not an ac-dc charger)?
Not that I necessarily trust company or professional info (a recent example involving incorrect Rostra supplied and website info being typical), but you seem safe with your Amazon return situation.
** Re batt capacity estimates & bullsh - I find them ok and on the conservative side (they need to cover 97% or 99% etc of their batteries), but I realise how they can vary.
My last estimate was a few years ago after losing my alternator. A 12 year old (10 year design) 38AH UPS battery and a one-hour trip home at night.
I estimated a 2 hour reserve for an ok battery with whatever load (~15A with lights etc). That gave me my initial confidence.
And then monitoring my dash voltmeter... With a ~0.3V drop over ~30 minutes I was confident I'd make it home - not that a battery cannot collapse at any point in time...
The best was the next morning after (battery) resting. A voltage check confirmed a discharge of ~50%, hence supporting my original 2 hour reserve-time estimate, and suggesting the battery was as good as new (at least to 50% capacity...).
Sure - somewhat variable and approximate, but I think my estimated errors balanced out (15A for headlights & lights & engine and occasional braking; was the 38AH rating @C15minutes) or C1, C10 or C20; how charged was the battery (it had been used for 1 hour cranking my car out of a bog); how aged was it; how conservative were the manufacturers specs?).
But with people even on this site thinking that simple component or equipment specs are bullsh (they fail to understand things like the test frequency or conditions & 95% etc rule), I can see the difficulty people have with batteries (SOC, temperature, age) - especially at loads different to the C rating. (LOL - especially when they think 38AH @ C20 means 38 Amps for 20 hours!)
[ ... end Incidental ]
I'm curious - will your wife be wearing your pants?