Building my DIY shower/bathroom, did I forget anything?

If you do the entire shower in one continuous layer of fiberglass it will not leak, but then again it will not flex either. It may, or may not, break then leak when it does flex. To avoid this problem, overlapping joints are the answer as noted above.

Main areas of concern; where the walls meet, where the pan meets the drain flange, where the walls meet the pan.

The grey vinyl liner as shown provides no wall waterproofing which you will need.