1. Im quite happy with my Heatkiller IV block. It cools my 2920x and 2600 with no problems. But if you have the EK block, use that for your 3900x and see if it cools it decently.
2. Ive never tried the new EK, Alphacool, Corsair, Phanteks GPU blocks. But nowadays, most CPU/GPU block (except the TR4 blocks) tend to perform close to one another with a small temp difference.
3 & 4. If you're happy with your EK rad + current fans, stick with it. Other wise, HWL for rads and NB eLoops or Darkside GT fans.
5. Personal preference.
Gaming Rig: (2) Swiffy MCP35x, UT60 + GTX 480 rad, (1) 360 HWL GTX, (1) 240 HWL GTS, HK IV 2920X @ 4.2 GHz, AsRock X399 Fatality, (2) NVidia 1080 w/ HK IV blocks, Gammix S11 Pro 1TB, Crucial MX500 1TB, 2TB Samsung F4, (4) 3 TB & (2) 5 TB Toshiba HD, CL STH10, EVGA 1300 PSU, Win 10 Pro
HTPC Rig: HK IV Stock 3700x, Asus Strix B450, D5 vario w/ EK top, 480 UT60, 240 HWL GTX, Asus 290 w/ XSPC WB 1000/1300, Crucial P1 500 GB, 12TB Ironwolf, 6TB & 8TB HGST, Enthoo Primo, EVGA G2 750, Win 10 Pro