I am building a wall that is 10.5' long and 28" high. There will also be side walls sloping down from both corners to enclose a patio. I would like to use blocks that are 15" wide, 5" tall and 8" deep, but there is no lip and it is concerning. There is about 6' of soil between my house and where the gravel backfill would begin and there is a very slight slope. Would construction adhesive between the blocks be sufficient for support along with the proper backfill and drainage?
If i understand the question, you want to use these blocks to create a retaining wall for your patio? the patio will be enclosed by these walls?
A 28" wall made from solid block is fairly heavy and might settle if not built on a footing or well compacted gravel base. You didn't mention what part of the country you are in. If you are in an area with ground frost, a footing becomes even more important. Seasonal movement can be substantial.
At 6 blocks tall, I would not recommend masonry adhesive. I think you should consider using mortar instead. Adhesive is fine for caps or just a couple of courses, but six interlocking courses should have mortar and possibly vertical re-bar if you want it to last more than a season or two.
Another consideration is the drop off height. 28 inches is a substantial drop. You should design some sort of railing or barrier so you or your guests don't step off a 3 foot drop to a broken leg or neck during a festive BBQ. Since I always cook with beer, it would be a concern to me!!!!
First the blocks you are describing seem plenty big enough for a 28" wall. You are talking about 7 blocks high. I would personally use a taller block but I like making the install go easy. Smaller blocks I believe are more work.
However I only glue the caps (top layer to the second to top layer). The other layers shouldn't need it. Either way the glue will last only a few years if you are in a temperate climate.
I also would not install blocks without a lip for any area that will be walked on that is that high. Having a lip or not really matters with the amount of frost your region has and the amount of traffic in the area.