They do have some certs from some not-so-famous lab, but they are also relatively cheaper, but there is a possibility that these labs may be not as strict as GIA.

Their current signature LoveMarque diamonds are graded by IIDGR or IGI. I would prefer the IIDGR certs because the diamonds should be newer and with laser inscription on the table of the diamond and with the  Hearts & Arrows image on the cert. Personally think that IGI stones are more like their old stocks, but I can't be sure.

Usually in-house cert means the jeweller grade themselves. GIA is an independent organization and they are the world's diamond foremost authority. To put in simpler, we can just say Wah Chan carries diamonds graded by GIA, and keeps these diamonds in-house.

As for IAG, my personal opinion is that their name and font has some motive of confusing people. Just my 2 cents. Again, sticking to GIA is the better choice if you are comparing AIG.