1. Had the PNF pressed the takeover button, would the PF have been able to re-take control?
Yes, pressing the take over button himself. Last one pressing has commands.
2. Is it normal for a crew at AF:
- not to be using standard calls (maybe even in English), but informal French throughout?
No. This shows much stress.
- to be addressing each other with the familiar "tu" in spite of the age and authority gradient?
Standard use. At AF the last in F/O will use "tu" in cockpit with the most senior pilot in airline if they're to fly together. It spares a few syllabs. After all, "you" can mean both "tu" and "vous", it's practical to decide for only one word and "tu" helps forming a team spirit. All crew uses "tu", except in cabin where "vous" is used. Sometimes F/A use "vous" talking to captain, mostly when they're white/grey haired.