The back does indeed look great!
The seam will more or less always be present-- unfortunate side effect of the printing process. If you look for "zseam alignment" or something along those lines in cura, you can set where it happens. I usually set mine to "Sharpest corner"-- that way it aligns with an edge and is hidden better.
Chep or teaching tech always has great tutorials on supports. I usually just turn them on in cura and leave them at default. Only thing Ill do is add support blockers from the left hand menu if I dont want supports in certain areas-- like at the top of the dome like I mentioned before.
Sometimes you can get away with no supports on horizontal pieces and rely on your machine's bridging capabilities, but if the line has any curve it will get messy like the visor did; straight lines only, up to around 10mm depending on your machine and it's abilities. There are bridging tests on thingiverse you can print to test out how far you can push your machine.
Layer separation is probably from printing the ABS without an enclosure. Drafts of air hit it and cool the plastic before it can bond properly. If this was happening all over I would guess that your temp needs to be upped, but since the back is perfect I would guess drafts. If this was in another material like PLA I would guess you wouldnt see those splits at all.
One thing that looks really good-- your layer alignment on the back is pretty much perfect. I have had some of the toughest time getting my z motor and leadscrew to play nicely and have had some prints with some wobble in them as the printer moves layer to layer