Coaster brakes can be adjusted with the wheel on the bike. As it's the only brake on the bike it's wise to have it checked over by a dealer if you're not sure about how to do it. You can first check if the brake is binding by turning the bike upside down and spinning the rear wheel. It should revolve freely with the brake off. If it doesn't, and you don't know what you are doing, take it to a dealer. You should be able to freewheel with a properly adjusted coaster brake.

Single speed single brake bikes are very limited in their scope (flame-war 1 on the way) but work ok in reasonably flat terrain. A single brake on the rear wheel is Neanderthal in terms of arresting bicycle motion. They're banned in the UK and Australia as the only brake on a bike because they frequently skid people under trucks.

The Downtube Nova has derailléur gears and two decent brakes. It's a recipe at least a hundred years old, and works well. You said:

'lets say the bike starts having issues with the gearing, is it possible for me to put an automatic internal gear hub on this bike and to keep the rear hand brake on so I can freewheel?'

The bike won't have any issues with the gearing, only with lack of maintenance. It will have normal wear over time, and all parts can be replaced or adjusted by any bike shop. You'll be able to freewheel, just like a normal, happy cyclist. All the brake parts can be replaced in minutes, without a PhD in rocket surgery.

Replacing a perfectly designed and serviceable, lightweight drivetrain with a heavier automatic gear hub would not be rational. (Flame-war No.2 on the way). If you must have that, get a bike with it fitted in the first place. If you're afraid of gears and brakes, get the bus.

Gears make life easier in hilly areas, for carrying luggage, or easing up on effort as you tire. (NB not 'tyre'). Single speed bikes suit the muscular and testosterone-ridden, and those who wish to be seen upon them evangelise about their 'purity' but single speed coaster-brake bikes are idiotically compromised as versatile bikes, and Dahon are pandering to the fashionably nerdy by selling them. (flame-war 3)

The Speed Uno is as much a fashion statement as it is a (compromised) bicycle. (flame-war 4)

Any of the folders with 20" wheels can be used for touring, including the Downtube Nova. There is nothing to fear with having two brakes, and seven gears or more on the back, other than fear itself.

A 20 mile tour is really not very far, the general requirements for touring being: a) some gears, b) some brakes, c) somewhere to put the beer cooler so it's not hanging off your back, and d); some comfort.

A bike with a rear rack, front and rear brakes, and seven or more gears may be the answer. Try the Downtube Nova, or any of the Terns, Dahons, Oyamas, Citizens and so on. Rear racks can be added to almost anything.

Put the Speed Uno on Ebay. A hundred beefy chaps will be queuing up to buy it.

Coaster brakes and no gears (you read it here first) are for kids, even grown-up ones. As bicycles, they are not versatile. If you want a bike that doesn't go, doesn't stop, doesn't do hills, and can't be used for touring; get a unicycle.

Flame-war No 5 will be along in a minute to utterly confuse the issue for you.

Hope this helps.