Learn Portuguese (Portugal): Eu tinha dado a nova versão ao criador. Tinha-lhe mostrado o protótipo. - I had given the new version to the creator. I had shown him the prototype.

In compound tenses like the pluperfect (an auxiliary verb plus a past participle), object pronouns must attach to the auxiliary verb, never to the past participle. Therefore, it has to be Tinha-lhe mostrado and not Tinha mostrado-lhe.

Could I place the pronoun before the verb instead, like Lhe tinha mostrado?

In European Portuguese, sentences normally cannot start with an object pronoun. Because the second sentence starts directly with the verb, the pronoun must attach to the end of the auxiliary verb with a hyphen. Even if you had kept the subject pronoun, it would still be Eu tinha-lhe mostrado because affirmative subject pronouns are not 'magnet' words that pull the pronoun forward.

Who exactly does -lhe refer to in this context?

It refers back to ao criador (to the creator) from the first sentence. The verb mostrar (to show) takes an indirect object for the person being shown something, so we use the indirect object pronoun -lhe (to him).

I sometimes see a one-word past perfect tense (like dera or mostrara). Could I use that here instead of tinha dado and tinha mostrado?