Hi Turtlebed,
This would be a preference choice and some people prefer one and some the other. The links about wool quilting and about the differences in the degree of protection between different covers in my last reply would answer both of these questions. If a mattress cover is designed to be used as part of the mattress construction (and isn’t a just a mattress protector) it would generally be fine although some would provide better protection for the latex than others. This is all part of the tradeoffs involved with every mattress choice.
It’s not the wool that would help a mattress keep its shape as much as using a suitable cover. I wouldn’t put loose wool inside a mattress cover. The quilting in a cover designed to be used with a mattress will keep the wool from shifting and migrating.
There may be some even less costly options yet in the lists that are linked in the tutorial post and in the DIY component list but either way I’ll be very interested in finding out what you end up deciding.
Phoenix