First, outstanding use of the forum features and for a first post, you’ve really crafted a clear and concise question. 
There’s a lot to unpack in your series of questions. You’re on the right track with your queries.
To answer one of your first questions regarding the use of bed adhesives. Much of what you may read online is based on old concepts dating back for when printers were hand-built high school science projects that used unheated glass plates. Simply put, anything you read on a forum or on YouTube you may want to filter for newer than two years because anything before that may have completely out dated information.
For adhesives. First, the term bed glue is a bad choice. Yes it does provide stiction to the build plate but it also acts as a release liner so that once the print is completed the adhesive layer facilitates the material coming of the plate. You will find this especially the case when using PETG on a smooth plate, good luck prying that off easily if you didn’t use glue. Counter intuitive? Yes. But that why I said the term glue is unfortunate.
When should you use them?
Smooth plate
- (PEI or PEO or “other” doesn’t matter) - Always use adhesive of some kind for easier release and better stiction.
Textured PEI
- PLA - Not really needed. You’re better off using brims
- PETG - Although it’s stated not recommended by manufacturers and listed as supported by others. If you’re using a high bed temp, you will want to use adhesive for release purposes not adhesion. However, you must and I mean must, allow the plate to cool if you’re using it at temps above 70 before you attempt to release a PETG print. I know this because I have damaged too many of my plates this way.
- PC - Not recommended because the bed temp needs to be at 100 but truth be told, I have experimented and you can use it but be prepared to chew up your plate. Glue will help release but not a lot.
No, your not doing anything wrong. And no, you shouldn’t stop printing things on their back. Instead, you’ll need to practice to get the “feel” of what works with each angle and filament vs what doesn’t. This as a much an art as it is a science.
If you can keep in mind this one concept, it may help. All an FDM printer is, is a hot-glue gun at the end of an X-Y plotter. As you drop the glue, gravity will take over, if you don’t have anything to hold it up, such as a preceding layer or support, the filament will want to droop. The sharper the angle the more droop.
The only workaround to this issue is to experiment with orientation. I realize that is sometimes not possible to do because rotating an object in one axis to ensure gravity is pulling the filament in the correct direction, will expose another side to gravity.
The remedy for this is to use model cutting and connectors to split your model. This will allow you the option of orienting different axis to achieve less stringing. The downside is that you have to reassemble the model but that too is an artform in that if you do it right and make the dowels fit tight through trial and error, you won’t need glue. I use glue because I’m lazy.
Here’s a recent discussion on that topic that may help.
And last but not least, “THE Benchy!!!”
Your models look good. Or as best as one can expect before tuning filaments. Search here on the many topics of filament tuning.
However, if I may suggest. Try out Orca Slicer, It has many features that Bambu Studio has and one of the best features is the calibration feature. It also has a built-in Benchy Primitive. Go through the tutorial page that has loads of suggestions. It’s one that I continue to refer to from time to time because it is always getting better.
