We experimented with the following metals:

Barium

Calcium

Copper

Lead

Potassium

Sodium

We wet metal loops and then put powdered samples of barium chloride and crystals of the other metallic compounds on the metal loops and put them over a non-luminous flame, causing changes in the flame colour.

Colour of flame with different metals

Barium: Yell0w-Orange

Calcium: Orange-Red

Copper: Green

Lead: White

Potassium: Lilac

Sodium: Yellow

How and what is it used for in "crime scene investigation/forensic science"?

By identifying trace elements, you can find out what the victim has been in contact with. E.g. in blood if you find trace elements then you can find out any potential murder "weapons" (in the form of substances).

If there's more than one trace metal present, is the flame test still suitable to identify the trace metals?

Some machines may have to be involved since under such a scenario the naked eye can only see one dominant colour. But I think even this has limitations.

Is the flame test sufficient to identify the trace metal(s) present?

I doubt it. Some metals do not change the colour of the flame at all. Also the amount of metal may not be substantial enough to produce a distinct distinguishable colour.

What is a "Mass Spectrometer"?

measures mass of electrically charged molecules (will try to understand and update)