If I take control of an opponents monster with enemy controller or change of heart, will I be able to sacrifice their monster at the end of the turn if I have ectoplasmer on the field?

Yes, since you control monster until the end phase, and both effects are resolving during the end phase, you would get to choose in which order the effects resolve. Hopefully, you'll chose to resolve ecotplasmer first 8^D

Yep, one of the many reasons why Eco decks are good... Darn those Two Man Battle Cells and Dark Factory!!!

Yes just incase you didnt get that you are allowed to do that...