Restaurant/Pub availablity

It depends. The popular stuff will get booked up, things that are either less well known, not as good, or both, will not.

You have said no to high end places, but there are still pubs like The Palmerston or The Broughton where you can eat above average food, that is in the Michelin guide but in a casual setting, for very reasonable prices, much cheaper than some other places in the UK (ahem London). But you will need to book places like this.

Inverness is much smaller than Edinburgh and Glasgow and popular/well regarded places that take reservations get booked up. One place that we like, which doesn’t take reservations is the Black Isle Brewery and Bar. It is very central and one street up from the riverside. They have large communal tables and serve good beer and excellent pizza.

We visited both Inverness and Edinburgh on a Rick Steves tour. My advice is to go ahead and make some reservations, either to places you see when out and about, or having your hotel recommend places and make the calls for you. In both places we tried just winging it, but ended up enduring long waits as a walk in, and/or settling for a tourist trap after getting hungrier and hungrier. You may not NEED reservations, but it's a better experience to have them.

Our experience was that reservations are definitely the way to go. Else, potentially long wait or just making-do with what was available. I don’t like to worry about stuff like that when on vacation.

I don't know if I'm stating the obvious, but you're much more likely to need to book a table at the weekend.

Likewise, because of the tourist numbers in Edinburgh, that would go double for that city as opposed to Inverness or Glasgow.

Booking a table to eat food in a pub seems like anathema to me, but may be necessary in this day and age of pubs masquerading as restaurants.

It depends how much you have your heart set on eating at one particular place. If you're out in town and one place is full, head for the next place, or just wing it and go somewhere that looks half decent. All of these cities are full of places to eat within easy walking distance of one another.