The frame data is referring to the MP in the LP>MP target combo.
So st.LP still starts up in 4 frames but the follow up to it (The MP) has a 6 frame start up.
Usually that’s what is being referred to in the frame data regarding Target Combo’s because the first button (The LP) is a normal st.LP, it’s the follow up which is different.
As far as I’m aware - No. If a move is +5 on hit, you’re able to hit anything that has up to 5 frames of start up. Any more than 5 and it’s not going to connect.