Can I build a 28" retaining wall using blocks with no lip?
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!!!!