Great report, and great to see you've got over those initial nerves and been bitten by the bug It is a very special feeling to see your boat afloat and to get into it and cast away from the bank, whether it is on a canal or into the open sea. It becomes familiar, but it never palls. When you're on your own boat, you are lord of all you survey.
I wouldn't presume to diagnose the engine problem from your description of it. Point is, once you got it going, it worked for the rest of the trip, so it's a sound engine.
All the other mistakes, we've all made. After more than 20 years of SIBbing, I did an hour the other day without remembering to raise my transom wheels. It was all low speed stuff on the river, but there's no excuse!
You can adjust both the throttle/twist grip tension and the steering tension as you go. I tend to have the steering tension fairly low when there is a lot of manoeuvring to do, and fairly stiff when I'm just cruising.
As for the motorbike thing: yes. I am on my 9th motorbike and 5th outboard and I still make the same mistake from time to time. Some people say sit on the starboard tube and steer with your left hand, but it seems more natural to me to sit on the port tube and use my right hand - until I make the fatal error of thinking about which way to turn the throttle. Thinking is often the problem!
Really pleased you had such a good trip. Keep it up. This is is a great hobby. One day you will be chugging awestruck under the towering cliffs of Devon or Cornwall, or creeping nervously at low throttle into Fingal's Cave, or seeing if you can creep through the Thurlestone at high water, but for now, every trip is a new adventure. You will see wildlife and meet interesting people. You will find places to swim. You will anchor on a quiet side stream and read, or listen to the cricket. You will have the confidence to stand as you steer, and nod companionably to passing narrowboaters and see their uncertain reactions. You will get a quiet satisfaction from every thing having a place and you knowing where everything goes. Enjoy every trip.