Hi Tiller,
Yes … every difference between two mattresses including the thickness of the individual layers themselves or of the mattress, the firmness of individual layers, or the type of cover and quilting can make a significant difference in how one mattress feels compared to another one. Every layer and component in a mattress will have some effect on the feel and performance of the other layers and components and on the mattress as as a whole and sometimes even differences that seem to be relatively minor can have a surprising effect on the feel and performance of a mattress as a whole. This includes the type of support system under the mattress and anything that is used on top of the mattress as well.
That would depend on whether the firmness or ILD of the 6" core in both mattresses was the same type and ILD of latex and on whether the cover and support system was the same in both of them as well but if everything else was equal and the only difference was that one mattress had an additional 2" - 3" comfort layer that used the same or softer latex then they would certainly feel softer yes. There is more about the effects of thickness in post #14 here and there is also more about the different “specs” that can affect pressure relief and the perception of softness in post #4 here as well.
That would depend entirely on whether the two bottom 3" layers in one mattress were the same type and ILD of latex as the 6" core in the other one. If they were both the same and all the other layers and components were the same in both mattresses as well then it’s not likely that most people would feel any difference (see post #2 here for more information about two 3" layers vs a single 6" layer).
That would depend entirely on whether a specific design was a good match for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences). While someone in a higher weight range will often do better with or prefer a thicker mattress … that’s not always the case for each specific person and there are too many unknowns, variables, and personal preferences involved for anyone to be able to predict whether any mattress is a good match for you based on specs (either yours or a mattress) or “theory at a distance” (see mattress firmness/comfort levels in post #2 here).
Post #3 here and the posts it links to also has more information that may be useful for those that are in higher weight ranges.
This would depend on whether the mattress you purchased was a suitable “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences). If you purchase any mattress that is too firm and it just needs some additional softness and pressure relief then a topper can certainly be an effective solution but the only way to know this would be based on your own personal experience.
There is also more about the different ways to choose a mattress and the different risks or uncertainty that can be involved with each of them in post #2 here that may be helpful as well.
Phoenix