Notes from my last trip here.

Don't completely dismiss the idea of renting a car. We (two women) drove from Tangier to Chaouen to Meknes to Rabat to Casa and had no issues with police seeking bribes (but did find police to be very helpful when we needed directions). Most Moroccan roads are in great condition, though in our effort to stay off the major highways, we did manage to find an incredibly scenic ~80km stretch of road construction that was a bit stressful but ultimately fun.

To answer your other questions:
>>Is Meknes worth only a day trip or would it be good to spend a night (or even two) here?<<

It was my second visit to Meknes (not sure how many times my Mom has been there) and I wish we hadn't spent so much time there. I just feel that it lacks the charm of some other cities, though it's advantage is proximity to Volubulis.

>>Can anyone recommend riads (or other hotels) which have free and strong guest Wi-Fi as well as offers a meal plan (either lunch/dinner or both included), without breaking the bank?<<

"Strong" wifi semes to be a bit of a challenge with the thick walls you'll find in most hotels and riads. I bought a USB modem (details here) and it was a help, but there doesn't seem to be a perfect solution. Every place we stayed offered free wifi.

We found that most riads include breakfast, with lunch or dinner costing extra. Keep in mind that riads are often just a handful of rooms (similar to B&Bs) and may not have staff onsite all day long. It's for that reason that many may not offer lunch.

We stayed at Dar el Menia in Fes and would recommend it, though lunch/dinner weren't included. We had great dinners within the medina each night, my food notes here.
[*]Is Tangier worth a visit (so we would do Casa > Fez > Tangier > Casa by train, with us flying into/out of Casa)?

I've offered a lot of comments about Tangier in this thread. My opinion: Yes. Other opinions: Mixed.