I'm not sure I have definitive answers for you, but I'll try to help.
For your first question, there is a Play All button available on the main menu made by AVStoDVD. When you select this, do the videos play one after the other or not? In the AVStoDVD Menu Wizard, under the Template tab, there is a box that is checked by default which enables the Play All button, but there is also another box that can be selected to Force Play All Button Selection. Have you tried putting a check mark in this box? This should make them play sequentially.
Your second question is more confusing. How on earth are you getting 65 tutorials accepted into AVStoDVD? The maximum number of titles that is allowed on a menu screen in this program is 16. Are you somehow generating more than one menu screen?
Each individual video that is imported into AVStoDVD is made into a separate title within the DVD-video output. The only way I can see getting a large number videos into the program successfully is to merge them in another program prior to importing into AVStoDVD. The tool you would use to merge them would depend on what format they are in originally, and all of the ones you want to merge together would have to have the exact same video and audio specifications in order to put them together.
There are more advanced authoring tools for DVD-video, most of which can accept larger numbers of titles (99), or chapters. Most of these require demuxed audio and video streams as input, like GUIforDVDAuthor, or DVDLab Pro. And you have to convert to DVD compliant streams before importing anything.
An alternative program, though not as good in video quality output would be DVDStyler. Be careful of unwanted "add-ons" in the DVDStyler installer. DVDStyler can convert as well as author.
And for your output size problem...you should divide your videos by playing time. Don't try to put more than 2 to 2 1/2 hours of video onto a single layer DVD. If you are burning to double layer DVD's, you can stretch that to 3 1/2 hours or so. Different movies vary on compressibility, so there is no hard and fast rule, but these should work as general guidelines.