If you are heading out of FRA on UA F, you will be able to access any of the Senator Lounges. However, for the lounge closest to your departure gate, you will head upstairs to the level for US flights and use the Tower Lounge. (You can not use the LH F lounges, as they are reserved for LH pax only.)

No, you can not waitlist for LH F .... but if you are on a full fare ticket, you can try to re-book.