While the 1968 GMC TDH3502 model city bus is in keeping with the classic vehicle theme, such as the Blossom's 1961 Chevrolet Impala convertible, the use of a city bus would only be on a local route. This would place Riverdale near Mamaroneck, NY and would not match the terrain in the area which surrounds Riverdale.
Mary tells Archie that whatever he and Veronica did or didn't find in F.P.'s trailer is inadmissible in court because they illegally entered it. This would only be true if they were acting as agents of the police, which they were not.
When Mary tells Archie that his evidence would not be admissible, her statement is wrong in two ways: As stated above, this only applies to evidence illegally obtained by police or their agents. The other reason is that admissibility only applies to incriminatory evidence. This is exculpatory evidence, so it would be admissible no matter who found it or how.