First, the coin you purchased is proof strike, mintage:10,000, 2011 value: $75.00. Font is as it should be. Text: PLaTA PuRA is shown on KM 494.3 and 494.4, 1991-1995 "Onza". Apmex picture is in the style of a business strike, KM 494.1, older 1982-1989 "onza". Mintage: approx. 2.5 million, value: $40.00 in Brilliant Uncirculated condition. No picture where shown in the catalog for KM 494.2 (1988, 1990-1991) or KM 494.5 (1991). Your's looks legit, apmex's is a contradiction.
I just don't follow you on the coin being flat and not fitting inside the coin case.
Further information can be found at the casa de moneda website.
Hanglong.