hi
1- no that is no need to excite all of them except for circular polarization or multimode use
2-i have doubt that we can use term of out of phase for this situation because this waves are excited and received separatlly(except for circular polarization)
3-i have not seen any application that excite modes in different time, but in different time there is not interference but in same time there is,
4-usually we are interested in first mode, because when we excite all modes in same time ,there is \ difficulties of several mode excitation (feeding) and inter modes interference