Is 2.4GHz viable for such a use of wi-fi?
Certainly it is
Not to mention microwave ovens
Umm I don't think you get enough leakage from microwaves to affect your WiFi. They are after all very carefully shielded against radiating.
I have recently installed a Ubiquiti UniFi mesh which covers both 2.4 and 5 GHz and it works very very well. Costs a "bit" more than conventional WiFi access points / repeaters but by golly it's worth it when you have a large number of people and difficult geography to support. The management software is great, and gives some fascinating insights into what's going on in and around the house.
In this particular scenario (a rambling two-storey guest house catering for up to 19 transitory and 3 permanent users) there are 6 access points, 5 upstairs and one on the ground floor, providing fantastic, seamless coverage. No latency, just smooth coverage (not a lot of enter-device traffic, it's pretty much all device-Internet). Most devices that aren't iPads, iPods, or MacBooks use the 2.4 GHz band.
Here's the astonishing part: at any given time there are 100+ (yep, over a hundred) visible APs in the neighbourhood. This is an ordinary suburb in Cape Town, not high-rise, not even high density cheek-by-jowl housing. OK, only the UniFi software can show me those SSIDs, and obviously many of them are unusably weak, but still... over a hundred, and zero noticeable interference.
I had another customer near the International Airport - right by the runway threshold - and conventional WiFi just didn't work. As someone else mentioned here, turn up the power and you only make a small difference for a short while. Installed one of these meshes and it was like night and day.