Usually, yes. A will can address property that was never transferred to the trust and may also handle issues such as nominations for guardianship. The exact role of each document depends on the plan and governing law.
No. Wealth is only one factor. People may use trusts for probate planning, privacy, incapacity planning, structured distributions, blended-family planning, or other goals.
The trust generally controls only the property placed under its ownership or otherwise directed to it. An unfunded or partly funded trust may not accomplish the intended transfer plan.