Lee is not a WLB in a 4-3, he is a heckuva athlete as a MLB/ILB, but he doesn't have the kind of speed a WLB in this scheme needs. So unless they were gonna draft Mosley and move him, no he wouldn't walk in and start here.
OK, and Zack Martin starts day one and so does Lawrence. Martin is a guy people see as a perennial probowl caliber player as well, and can supposedly play all 5 OL positions. And getting the QB is still the most important part of a defense, and Jernigan wouldn't help that as much as Lawrence.
Pass rusher >>>>>>>>>> safety
Lee would be fine at WLB. He can cover and is a tackle machine, perfect in this version of the 4-3. Also, Mosley was projected at all 3 of the LB positions by several draft publications. He can also play the WLB if needed.
Martin is a fine player, and so is Lawrence, but I take 3 starters over 2 any day of the week. If you project that over several drafts, then you see a much bigger difference over time. The reason they were desperate for talent is recent bad drafts.
And I also think Spencer is much more of a typical RDE than LDE. First, he is too small for LDE. He is a good enough rusher to play RDE, though not as explosive as Ware was. Seattle won the SB with a rotation at DL which did not include a Ware type player.