Forum pro tip: Questions first, background later.
Are there any serious, long-time runners/climbers here who could progress well in both sports?
Do you guys know of a good way to mix running and climbing throughout the week that has worked well for you?
Yes, schedule your harder climbing days when you’re fresh(er), perhaps on rest days for running and vice versa.
How do you go about peaking for a race/project?
Tapering is very personal and varies per person.
…how much volume do you think one can do with this approach? Do you think 8+ hours of running and 3 times climbing per week is something that one can maintain for a longer period of time?
Which “one”? Training load depends on training history. Three hours might be a lot for one person, and 12 might be too little for another. Most people will improve on eight hours; Kilian Jornet would lose tons of fitness.