You were braver than I would have been! There is that restaurant at the very top of Takao San, but I would not recommend it for anything but the most basic of dishes - getting fresh supplies up there cannot be easy. It’s a great place for snacks or pre fabricated noodles using pre-made stocks and sauces, but for tororo soba you really need to be down on ground level at one of the specialist restaurants near Takaosanguchi station (Takahashiya is one of the longest established examples). I LOVE tororo soba, but I would have done what your wife did and gone for the udon - a much more robust choice, way harder to screw up.

6 hours up and down seems a reasonable expectation to me. That’s how long it takes us (it’s about 7km - 4.5miles - up and down without the cable cars)
We avoid weekends, and tend to have an early lunch near the station. That way we are ready to climb at about 12am. By the time we have made it down again it is about 6pm and the sun (in March/April or October) is setting. Bath and food at the Onsen and we head back at 9pm.
This has been our routine since our child was 5. I assumed those without kids fascinated with finding frogs in rocks, etc. would complete the course a bit quicker.
Hardest climb was when the kid was nearly three and we took a stroller. Managable, used the cable cars of course, but gruelling.

Anyway, thanks for the report back!