That's a good thought, but consider population density and road density in Europe.

In Scandinavia, yes, sticking to back roads means you get nice scenery and a calm ride, even if it takes longer to get somewhere.

In Germany and France (though I have only ridden small parts of France), sticking to back roads means being stuck in commuter traffic on single-lane roads with no opportunity to pass, for hours, swearing at trucks and hating yourself.

Unless you're in a relationship, I would suggest separate tents. Also, you need a thermarest/mattress for inside the tent. I highly recommend doing a long weekend trip somewhere with your full gear, camping etc., to check how it works in the real world.

Couchsurfing, booking.com (your phone will have free roaming from next year!), a Hosteling International card, and a good attitude.

You don't have to eat in fancy restaurants, but a lot of the time local streetfood is excellent and cheap. Within the same range as buying supplies from a supermarket.

If you are camping, I would suggest learning to cook over an open fire - much tastier that way, and you are not dependent on finding gas. But yes, most countries in the world will have standardized campingaz-type cylinders in supermarkets and outdoor supply stores. Borders should be no problem.

Absolutely it is. It's cheap and gorgeous. The Vrsic Pass is one of Europe's great biking roads. I also loved Maribor, and Lake Bled is something you have to see once in your life.

Until you get to Serbia there will be no borders at all. South of that, you will probably get a stamp.

Earplugs. Cheap foam earplugs are the best cost-to-value thing a biker can have.