So I am leaning towards Suntoys as the maker and not Summer. It also fits better to the information from Kimmo, Dave and Marks small diecast book about Suntoys.
With the added information I would lean that way as well.
Since juantoo3 already has both Greenbrier, Summer and Suntoys each listed as individual entries in his posted small scale master listings. How is it that you would update the listings - in this case -as to 'maker' 'factory' and 'trade' names you have mentioned elsewhere?
It is possible the name Greenbriar came up before. I did not vet every name on that list, the names were provided by a long list of individuals...one could say Eric "tricked" us into a game and we all tried to outdo each other coming up with obscure names, but we did have to supply images to support those claims. Names on the base were acceptable, so my guess is someone not unlike yourself showed a model with Greenbriar on the base...which got it added to the list. As I pointed to with Ja-Ru and MarzKarz, both of which are well known and understood examples of distributors being mistaken for makers, it does happen, particularly with the obscure cheapie brands that get marketed in a long list of trade names. I think the work of Kimmo, Dave and Mark is very thorough up to the point they published, but it has been about 15 years since then and nobody I know of has kept the detailed tracing of every one of these obscure (no-name, generic) Chinese makers. I have peripheral interest, but most of my focus has been on vintage stuff (so I've been wrapped up the past few years in Hubley, Budgie, Benbros, Konami and pre-production stuff).
To answer your question directly, if we can presume at this point these are Suntoys models, and if I can presume Suntoys has taken over the Summer production facility, then Suntoys would be the factory and likely also the maker. Greenbriar would be the distributor in any case, and likely contracted Suntoys to produce the models wholesale which Greenbriar then wholesaled to (in this case) Dollar Tree for retail sale. That would be my best guess, but there are variables along the supply chain that can affect the answer. If we can determine that these "Greenbriar" models are the same or substantially similar (possible minor modifications) to known Suntoys castings, then I would be comfortable calling them Suntoys and that is how I would categorize them. Your mileage may vary... Typically the trade name is on the front of the packaging...apart from the number 3 on yours and the word "pack" and the French equivalent, I don't really see any other distinguishing trade dress across the packages. In that case I would look on the back to see if there were any information, but so many of these are blank, and I am presuming yours are too. That leaves little to go on, but Greenbriar would potentially be the trade name as well, or "3" or "3 pack" if it weren't that the cars are packaged 3 to a pack. The cheaper the generics get, the less information to go on. It has been suggested on more than one occasion this might be to skirt licensing requirements...can't enforce infringement if you can't trace who actually made them...
I am still hesitant about the NSX - as it does not appear as a previous Suntoy casting (or Summer). And it is the one embossed with the Greenbrier name.
It really is not at all unusual. If, by the company it keeps (packaged in the same pack) it is with Suntoys models (I'm still leaving a sliver of doubt), I would be inclined to think it is a Suntoys model, in spite of what is on the base. Bases get modified routinely. Take a look at the connection between Yatming and Imperial or Road Champs. Granted, there it was paper labels covering the source...in this case Yatming...but once out of the package and little fingers picking off the label there is nothing to distinguish how the car was released to market. That would be a fair argument. But what of other times when models were / are contracted and the base *is* changed? Can't really point to Ertl-RC-JL because that wasn't contracting, but I can point to High Speed-Grell-Malibu International and likely Schuco. High Speed models sold as Grell had the Grell company logo on the base. Likewise with Malibu International...and I would not be the least surprised if the same is true with the Schuco issues of High Speed models. So changing the base is not unheard of, and with a bit of looking around I am sure I can come up with more examples. Bases get changed all the time, and is probably one of the easiest (certainly most common) modifications made to tools. So having the Greenbriar name on the base is good, certainly something if nothing else is available, but not an absolute identifier...as we see once again with Imperial and Road Champs.