How to mix cream to increase its fat percentage?
I think it is quite difficult to increase the fat content of the cream with your proposed ingredients.


Philedelphia cream cheese has about 35% fat. 
Mascarpone cheese is made from curdled cream. It has a higher fat content than cream due to the removal of whey. There can be about 44g fat in 100g mascarpone. The real fat content might differ. The only thing that I know for sure is that mascarpone has a fat content of 80% in the dry product.
Crème fraîche is just soured cream and has the same fat content as cream - 35% (see also the wiki article in Dutch). 
Quark (full fat) in Germany has 52 - 68% fat but I think the regular quark has less than 10%.


Have you seen double cream / crème double (40% - 60% fat) in a grocery store? It's just cream with higher fat content. This could suit to your ice cream.



EDIT: I found an older question here on cooking.SE: Is there any way to make single cream to double cream? and Could I add butter to single cream to make whipped/double cream?



I'm wondering if it's possible to increase the fat content of cream if the cream is drained in a cheesecloth...