I think your question is really two questions:
-a Jew wishes to heat his kosher food in his nonjewish friend’s oven
-we wish to heat chometz in an oven which is already KLP and are trying to avoid having to kosher it all over again.
In the first situation, I would clean the oven and then do self-clean cycle or set at max heat for a while (never had a nonjew or nonobservant jew make unpleasant comments, of course I always offered to share the food), but I am aware of many leniences (even beyond what was posted). The leniences come from the difficult reality of our ancestors, who sometimes used the local baker’s oven to cook.
The second is a little bit different, because the oven is yours.
Personally, I am aware that we have to observe Pesach for eight days only and no more – that’s where beer and whisky and restaurants come handy :-))
Ver Bin Ich, I am quite sure our grandmothers and grandfathers themselves would not eat in their own kitchen. It was not easy life, I am sure they would tell us we should realize how blessed we are, not being forced to rely on certain leniences.