Adding fingering causes slurs to distort

Slurs normally go outside fingeirng, and fingering is normally much closer to the staff than this, so this would normally not produce results like this. It seems those numbers are not actually fingerings, so perhaps better to add them another way, such as using staff text (and you can use Alt+Right when entering them to move from note to note as easily as entering actual fingerings). Or just adjust your text style to keep them closer. Seeing the actual score is always better than a picture for giving suggestions. But, to answer te question directly, if not necessarily with the most appropriate result, you can get MuseScore to allow collisions for any element by disabling autoplace for it - press "=" or use the Inspector. In your case, disabling it for either the slur or the fingering would probably be fine. Stacking order is not relevant, that's about which element is actually drawn on top of which when overlapping.