i'm not sure i followed that correctly but it sounds like you removed four fish (one of which didn't make it) that were symptomatic of what you diagnosed to be marine ich into quarantine (separate system) for three weeks then moved into a frag tank off the same DT system for two weeks leaving two fish in the DT system the whole time. is that correct?
if so, i don't understand your goal. were you assuming there was an outbreak due to stress between the inhabitants not getting along or picking on each other and you were trying to reintroduce them in an order to allow the skittish an option to get their territory prior to the more aggressive added later on?
from what i gathered in your reply i didn't see any mention of treatment used so the ich is certainly still being carried by all the fish. but certainly more so because the DT was never left fallow to allow the parasite to die without a host for at least 8-12 weeks. with such few fish and at a critical juncture at your build, it'd be a great time to develop and begin a proper quarantine protocol with research through the great threads on here and with the hopefully insightful help of your friend/shop-owner. in this way you ensure the healthiest specimens make it to your tank and can work through a fish list diligently adding the appropriate species at a time.
if you have decided against quarantine or prophylactic treatment sorry if i missed that post and this is a bit of a rant but wanted to make sure you were not confused with the likely outcome of your actions and hard work.
