You will need to put your laptop in your carryon / rollaboard to get through security.

75mins is going to be tight, depending on security at the connections center. I don't think that going through passport control and through the landside connection / security would be quicker unless you have EU passports (and even then it would be a toss up).

UA will put you on a later flight if you miss it. This might mean connecting in IAD or ORD (I haven't looked up the timetable), and a significantly later arrival. Another hitch might be that you dont get BPs from LH at MUC for your UA flight - seems to be a lottery on this - in which case you'll need to check in at th connection center and you will be after the checkin time by then I would think.