I do partial mashes and have added DME as a late addition and also at the start of the boil. In terms of taste and color, I don't see any difference and this is on final products with SRMs in the 5 range, so fairly light in color. I also found no difference in the fermentability of the resultant worts.
The small color change is coming from
Maillard reactions, not really the same as carmelization.
My understanding is that DME has not been put through a hot break. It is
post-mash wort (pre-boil) spray dried into powder. A second reason for believing this is that when I boil it, I get a hot break.
How much stuff is in your wort when you are hopping definitely impacts the amount of hops you should use. Beersmith will show you this quite clearly. I made a simple recipe in beersmith moving the DME from 60min to 15min to show the impact (see screenshots below).
Late extract addition yielding 36 IBUs from 50g EKG:
Extract added at start of the boil yielding 27 IBUs from the same 50g EKG:
Personally, I switched from doing late extract additions to just putting it all in at the start of the boil for convenience because I could not perceive a color or flavor change from trying it both ways. I also did not like that the late extract addition interrupted the boil making my hop times all wonky.
My experiences shared here only apply to DME. I have not done the same comparisons for LME, which I no longer use because it is so much less convenient than DME. Cheers!