A: May an altar server incense the priest and the people when a deacon is assisting at mass? Or is the incensation proper to the role of a deacon? (See GIRM 178).
Q: The vocabulary around acolytes is particularly difficult to follow in the GIRM. At times it is definitely referring to an instituted acolyte, but other times it seems to refer to an altar server. In fact, the word “servers” appears only once in the GIRM, both in the US and in Canada, and that is in 339, where it speaks about the local custom of vesture. The word “servers” was added by the conferences; it has no Latin equivalent in the typical edition. Otherwise, one logically assumes that many references to acolytes are references to altar servers.
The exception, of course, is in the distribution of communion and the purifying of vessels. In those cases, the GIRM strengthens the word “acolyte” by referring to “[duly] instituted acolytes” (162, 191, 192, 247, 249, 279, and 284.) The expression also appears in a section treating this ministry (98, 100), which embraces some duties that may be carried out by one who is not duly instituted.
To make the distinction, the GIRM sometimes refers to “acolytes and other ministers” – such as 120, 139, and 140, paragraphs that concern actions such as carrying the processional cross and setting the altar. One need not be instituted for this purpose.
The same applies to the use of incensation, which is explicitly mentioned in 187 and 190 as an action that a [non-duly instituted] acolyte may perform.
You are asking specifically about GIRM 178, which says “the Deacon himself or the acolyte incenses the Priest and the people.” Incensation is not one of the areas restricted to a “duly instituted acolyte.” It couldn’t be because instituted acolytes are fairly rare, and we use incense more commonly.
I conclude that the incensation of the priest and the people during the preparation of the gifts is not a ministry “proper to a deacon” when a deacon is present. However, in practice, I generally do invite my assisting deacon to incense me and the people. He’s more skilled at it, and the ceremony flows smoothly. But the practice of having an altar server incense the priest and people when a deacon is present is not contrary to the norms of the GIRM.