Can a changeling mechanically imitate/impersonate a warforged successfully/convincingly (presumably the changeling could add armor and the like to his/her "costume" appearance to help with the disguise)?
Yes. Changelings can impersonate warforged.
The Wayfinder's Guide to Eberron (p.61) and Eberron: Rising from the Last War (p.18) both describe the change appearance racial feature:
You decide what you look like, including your height, weight, facial features, the sound of your voice, coloration, hair length, sex, and any other distinguishing characteristics. You can make yourself appear as a member of another race, though none of your game statistics change.
Changelings are good at imitation
Part of their racial feature grants them advantage on maintaining the ruse
Even to the most astute observers, your ruse is usually indiscernible. If you rouse suspicion, or if a wary creature suspects something is amiss, you have advantage on any Charisma (Deception) check you make to avoid detection.
Clothes do not make the warforged.
Warforged are one of the few races frequently depicted without clothing. So being naked isn't a give away nor would being clothed as some do choose to wear clothing.
Your clothing and other equipment don’t change in appearance, size, or shape to match your new form...
Natural Armor vs Worn Armor
The natural armor of the warforged isn't equipment, and would be fair game in the same way the scales of lizardfolk are. However, they can wear armor by "incorporating it into their body" (E:RftLW p.36):
To don armor, you must incorporate it into your body over the course of 1 hour, during which you remain in contact with the armor. To doff armor, you must spend 1 hour removing it. You can rest while donning or doffing armor in this way.
Armor worn this way is still equipment and not valid for being replicated by a changeling.
Warforged are valid targets of impersonation
Slightly diverging from the previous editions, which had them as living constructs subject to different rules, in E:RftLW (p.35) they are explicitly described as living humanoids subject to the same rules as other races:
Although they were manufactured, warforged are living humanoids. Resting, healing magic, and the Medicine skill all provide the same benefits to warforged that they do to other humanoids.