In answer to your first question, yes, you would apportionate the amount of earned income received from both states, the amount showing on the W2 from MD and the amount showing on the W2 from DC if that was your only earned income. Because you may have additional earned income besides the wages, you would need a precise allocation for all those earned income items. see HERE for information on how that's done.
As far as your second question is concerned, there are various adjustments to income that you may or may not have had on your federal return , that must be added back for Maryland purposes. These are called Non-Maryland income and Adjustments. see HERE pg. 7 number 11
And finally, for your last question see HERE for preparing nonresident returns in Turbotax.
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