To begin with, many of the storage issues raised by the intrinsic vice issue of cels are not as applicable to paper products. In short order a cel will damage the bag in which it's stored, and so additional care must be taken to buffer or absorb the chemical products that cause this reaction. (In time this same reaction will do the same thing to the cel itself if care is not taken promptly.) Paper is much more stable, and the only issue is the acidic content that most varieties of inexpensive paper contains due to its manufacture from wood pulp. That acidic content gradually turns paper yellowish and in time makes it brittle. In general, careful storage away from sunlight and contact with higher-acid materials (like celotape adhesive, post-it notes, and newsprint) is enough to conserve such products. This is also why the best practice is to store dougas and backgrounds separately from the cels with which they came. (And keep good records so you can reunite these materials at time of sale or trade.)
Some of the practices that you mention come from comic book collecting, where the covers are of a different paper from the inner pages, which are often newsprint and high in acidic content. Thus over time the newsprint pages yellow and damage the cover art. So it is crucial for such items to put a glassine sheet between the insides of the covers and interleave the books with microchamber paper. Also to use a buffered storage box.
Fortunately, animation art is not done on newsprint, so the acid issue is not as extreme. Still, given that many gengas and dougas from the 1980s and earlier are visibly yellowed, it is a good idea to use microchamber paper inside the bag and to be careful with what you use as a backing board.
As for your questions:
1) To store items like this, is better a buffered interleaving paper between the mylar and the sketch?
[I personally use archival-quality polyprophylene bags rather than mylar, as they are inexpensive and can be used for cels as well as paper goods.] I do not see that there is a need to put anything between the sketch and the front of the bag.
2) To store gengas/dougas and considering that they has different colours, is better unbuffered interleaving paper, right?
I've heard anecdotally that the thin colored paper used for correction (shuusei) sketches is higher in acid than the thicker white paper used for layouts, gengas, and dougas. So I use a non-acid (but unbuffered) backing board to separate colored/uncolored sketches. Colored sketches on one side get one sheet of microchamber paper for every three sheets, and white sketches on the other side get one MC sheet for every five sheets. That quantity is just my personal habit and is not based on any experimental data with paper aging.
If you are concerned, you might contact the manufacturer, which does test its product in various ways and may have a more scientific answer to this issue. But I have not seen any significant change in the condition of my paper art in the twenty-plus years that I have been collecting, so I think that my practice is at least adequate, maybe overly cautious.
3) How often do you change the buffered / unbuffered interleaving paper?
I have very rarely changed interleaving paper, and only in odd cases in which the sketch had a strong scent, most often of mildew or cigarette smoke. If the interleaving paper picks up this smell, I suspect that it is saturated and needs to be replaced. Otherwise, the work that the microchamber paper does seems to be adequate to prevent damage in the long term.
I would be happy to defer to other experienced collectors who have paper-storage solutions that work for them.
