Hi Vitoria,
I think if I was in your shoes I would lean towards a mattress protector (for the top) and not an encasement that protects on all 6 sides against dust mites and bedbugs. Dust mites need to eat and they feed primarily off dead skin cells and if you have a protector you deprive them of their main food source and dust mites would not be a real concern … at least for me. If a mattress encasement has small enough pores to protect against dust mites … then it would also affect ventilation and breathability. This would be particularly true with the membrane type of “waterproof” protectors or encasements which are vapor permeable but not really breathable to the same degree as the other two types of protectors that you can see in post #89 here. While the sleeping temperature of a mattress depends on many factors (see post #2 here) and it’s not likely that just one of these factors (a mattress protector) would lead to sleeping hot on a cotton mattress … I would personally tend towards either a cotton or a wool protector depending on the degree of water resistance you were looking for and would lean towards a thinner wool mattress protector like the St Dormeir because wool is naturally resistant to dust mites and holds moisture rather than allowing it to go through and would also help prevent moisture from going through into the mattress and deprive any dust mites from the other essential they need to survive which is relatively high humidity levels in the mattress.
All of it no but but some of it yes. Again … the breathability and temperature control of a mattress depends on many factors but there is no doubt that cotton or any natural fiber would breathe better than any foam including latex and the layers closer to the top and how you sink into them have a greater effect on sleeping temperature than deeper layers. Having said that … latex is the most breathable of the foam materials and having the cotton and springs underneath would be a cooling factor compared to having more foam underneath (such as an all latex mattress) so once again it’s a matter of degree and perspective and it’s probably fair to say that most people don’t sleep warm on latex (unless it is softer and thicker and possibly with a less breathable cover, mattress protector or sheets or with other factors involved).
Quite frankly … I wouldn’t be particularly concerned about either one. Dust mites wouldn’t be an issue for me with a mattress protector that could be washed and I also wouldn’t be concerned about the latex becoming crusty which only happens over many many years if you have a cover on the latex (see this 40+ year old latex mattress in the video here which only has minimal crusting). The crustiness of very old latex mattresses is a result of oxidation as it interacts with ozone and ultraviolet light primarily not dehydration. The latex will soften to some degree over a longer timeframe but not dehydrate (and of course with a topper you have the option to replace it if necessary or even change it if your comfort needs change). Having said all that … you are asking something that I don’t really know and I haven’t seen any information that has compared these two factors directly (even though neither would be a concern for me) and since Jim has worked with both for many years … I would defer to his knowledge and/or experience on this one.
Hope this helps 
Phoenix