Thicker is not necessarily better. Bellows board usually has to be skived on the outer edges or else the outer hinges and leather topruns will deform into the joints. The thicker the card the more pronounced the skive has to be. Another issue is that thicker card will make the bellows appreciably wider when closed (can be a problem fitting it in cases).
1. Would it make a appreciable difference using cardboard that is thicker than 1,5 mm or if cardboard with this dimension is so stable, that thicker one would not make any difference (exept for beeing thicker)?
Your question doesn't really address the issue. There are many grades and types of card. What you want is something stiff, durable, fairly thin, easy to cut and glue to, readily available and reasonably priced. Wood pulp has natural lignins which are acidic and cause the board to deteriorate quickly. OTOH, "good" quality wood pulp boards are treated to neutralize the acidity (then called acid-free).
2. Is there any appreciable difference in using wood pulp cardboard compared to using (pure) cotton cardboard?
Not necessarily so. Better quality ragboard has linen content in it (linen is twice as strong as cotton and lint free) and/or hemp (hemp is eight times as strong as cotton as four times as durable).
Ragboard is, as far as i know, another word for pure cotton cardboard
Quarrel paper? Paper in dispute? To have a fight over?
(like rag paper ... in german it is "Hadernpapier").
-- Rich --