Hi jojo,
1.2 lb foam is very low quality and 1.5 is low/mid quality.foam and along with the Bonnell coil (which can be good but are less costly than other coil types) this would be a very low cost/quality mattress. There is over 4" of the type of foam here that can cause softening and durability issues and the more rapid loss of comfort and support that goes with it. I would tend to avoid it.
That depends entirely on the materials that are in your mattress. It would surprise me if the mattress you are looking at would last that long even although your lighter weight works in your favor and also allows you to use softer foams. Better quality materials (such as latex) could last twice or three times as long depending on many variables. Post #4 here along with post #2 here and post #3 here and post #8 here talk about the durability of a mattress and how it is relative to each person.
Phoenix