Can you go in, before then, for some one on one so you can enjoy your machine and feel good about it before you go on Monday??
It's good you are going for your lesson is so soon. At least you don't have to wait too long to hopefully get some solutions. Though if you can arrange the sooner, it would be nice, as above.
When you are sewing two pieces of cotton together with nothing fancy (as you would for seams), you should be able to sew with the threads meeting in the middle of the two pieces. If your dealer is telling you otherwise, I'd wonder two things ... either s/he doesn't know their stuff OR s/he has misunderstood your problem and questions.
Here's a suggestion ...
First, turn your machine off and walk away from it for today.
Later tonight or tomorrow, go back and start fresh.
Take spool and bobbin off/out and re-thread totally. Even wind a new bobbin. Maybe even start with a brand new spool of thread.
Then use two different colours of the same thread for bobbin and spool with your fabric. Have good contrast between the three so that you can easily spot what is going on. The fabric is ideal if it's a plain and white/pastel so your writing will show. Write on the fabric the brand/weight of your threads. Set the thread tension dial to 3.
Do some rows of stitching on two layers of the fabric. Mark on the fabric with a pen/marker the tension setting. Check how the threads are meeting up. Too high? Too low? middle?
Refer to your manual which will tell you how to adjust the tension so that the threads meet in the middle.
More rows of stitching and label again. Check.
Continue on ... and hopefully you will make some progress.
Then take the samples into your dealer to discuss.