Sorry it took me a bit.
First of all some of what i say is a guess, it is an educated guess but still.
On the tail light - it's your car you always get to pick - it's just that your pick may have consequences like you have to pay the difference. I think you're right to prefer the recycled part. Often times new lamps stand out like a sore thumb, though it is more common with headlamps or clear lamps.
On the estimate - i do know how the systems work and I'm a little surprised you can see this kind of estimate at all. All of the systems have a preliminary and final estimate sort of function. Maybe since you cant see the actual estimate the dollar amount is something the insurance companies portal is deriving from who knows where. It's also possible, maybe even probable that the 2400.00 estimate has been split into two estimates one for additional damage and one for corrective damage.
So for instance if the tail light is broken and it is thought to be from the accident and the other shop was not paid for a taillight - then the insurance company would owe the new shop for that and cant make the old shop pay them back. if on the other hand they charged to repair and paint the taillight pocket but did not do so or did so poorly then the insurance company can charge them back. The estimate would need to be split for accounting purposes. I do not think the estimate went from 2400.00 to 800.00 for a pricing change for the same items.
These shops are probably rated and ranked heavily on metrics like cycle time and satisfaction, neither one of them wants you unhappy and neither one of them wants your car there a long time, sometimes though and most of the time with something like this in order to to complete the happy part it takes time. Much like repair facilities, i think sometimes the customer facing part of it is not as technically savy as we might think or hope.
if you can get your hands on the final bill from shop 1 and the corrective estimate or estimates from shop 2 and want to send them to me either by PM or by PMing me for an e-mail i'll try to refine my opinions / you can redact or scrub them of personal information.
RE-frame damage there are 4 basic types of frame damage. 2 of them are almost never present in a repairable unit body car those two are twist and diamond. Twist and diamond affect the center section of the frame. That leaves sidesway and sag. In very basic terms sidesway is right left and sag is up or down. i think if the gaps are good on the trunk and rear doors then its unlikely there is actual "frame damage". Cracked seam sealer is an indicator of movement in the structure, things like the trunk floor and rear body panel are likely Body-S or body structural parts, this stuff can move a lot in collisions, the real question is did it return to its original spot and is anything other than resealing necessary?
This sounds like a fairly minor collision you're dealing with, but there still could be secondary damage somewhere. if you watch some crash test videos you can see how damage can travel through a unit body.
