Would Mereka ingat Andi have the same meaning?
Yes.
why do they use mengingat instead of ingat?
That's how it's said in the formal style.
Hard to hear that one
Remember as in are they missing him because he's gone for a long time? Or they're remembering him because he's dead?
They remember Andi because he went to the bookshop without pants.
Its just simply remembering. U may use this sentence in every kind of situations.