Learn English – How to correctly use the possessive apostrophe with a parenthetical citation

Don't write pairs of parentheses back to back (don't do it)(no, really!). Either you can put on of the two items in the main text:

Treatment A contained significantly greater mass than treatment B (p = 0.001), as show on Figure 1.

or use the semicolon, as you proposed:

Treatment A contained significantly greater mass than treatment B (p = 0.001; Figure 1).

I can confirm that in spoken word the sound of 's comes after the phrase. I don't have any authority on this area of English but I would probably write the sentence as "Billy (who goes to my school)'s favorite game is tag."