Answering your question in reverse order:
2. The U-shaped trap is absolutely essential. Otherwise you woould have sewer gas in the house.

1. A flex-trap may or may not be allowed by code; I would not recommend it. You cannot ever run a snake through a flexible bend, and shower drains are likely to need a snake sooner or later. The solution is to remove all the stuff you described, back to the stack, and rebuild it. This might be time to call in professional help.