i'm not sure if diamond shops offer a magnifying scope for customer to inspect but you can buy 1 cheap on ebay from China, and bring along a flashlight, just aim the light on the diamond and use the scope, you can see the reflections & clarify clearer.

certs are only for reference & maybe value proofing if you plan to sell, in the end still rely on your own judgement.

i'm sure if you check around 100 diamonds from different shops & different specs, you should be able to tell which ones are better, and also an idea on the of local shops.

you can buy from online like someone suggested it'd be cheaper & more options, but make sure you get it from really reputable shops & reputable courier service like DHL with full insurance who takes care of all the handling & tax for you. but you have to bear the duties & GST & handling fee. always a risk buying online.

remember diamond is not an investment like Gold, because synthetic diamonds are very common, thus the value rarely goes up unless it's some rare colored, very rare high carat or extremely well cut diamonds (usually by Brands).

you want size or you want clarity, your eyes can probably tell the size & cut difference, but can your eyes tell the clarity difference? do you have a habit of using a scope to admire the diamond?

since you can't bring her with you, good luck trying to find a ring she likes. if she not into diamonds, probably easier to pick.