Darius Q change will make it too easy to avoid, making it deal no damage, not apply bleed and deny access to his heal due to how bad the handle damage part is. Nerfing his HP regen would be a better change because he has a really high health regen both at early levels and at high levels (10 hp per 5 at lvl 1, 26 at lvl 18). I understand that his Q heal is very strong when used properly, but his Q already has a lot of counterplay, being delayed, short ranged and the handle part is still very big. Making it bigger will make his laning phase against melees auto lose, because they will always walk into you and take no damage from the handle, making you lose every trade due to not getting the damage, bleed and heal. Since he has a heal, he shouldn't have great health regen to top it all off, since it gives him a lot of hidden power during the laning phase since he passively out regens everyone, making people feel like his Q heal is too strong in lane when i reality most of his sustain comes from the fact he regens twice as fast as you.
The E change is fine, I honestly don't even think it needed a slow in the first place because the first thing you're doing afther pulling is slowing them with W, but a 40% slow will make it easier to get out before he can slow in some cases, which is fine. What isn't fine is his W. At lvl 1, you have the maximum amount of damage and the maximum amount of slow that the spell can give. Leveling it feels bad, it doesn't feel rewarding to have it's CD reduced by 1 second, since you're generally building cleaver / visage for CDR, so you don't even need it to have a lower cooldown. This makes Darius max E 2nd, making him get 25% armor pen in the mid game, giving more damage on his abilities and autos, giving him hidden power since he's getting free armor pen and not losing on damage on W. Also, the CDR on E is much more valuable than W, because if your E is on CD, you won't W them.
His W needs both of these changes of these changes:
1) 110-150% total AD damage from lvl 1-5, making you lose damage if you keep it leveling one, but reward you for maxing it out with a little big more damage.
2) 50-90% Slow from lvl 1-5, slow duration 1 second lvl 1 to 1.4 lvl 5 limiting his sticking power if you don't max it 2nd, but giving you a longer slow as you level it up without it being too strong.
As for Mundo, I really like the E change, espicially since it counts as an auto attack reset, greatly improving his trading power in lane and gives him a better scaling into late game with an HP scaling ability. Also you can use it in lane, wait 4 seconds, auto them then E is back up and E auto them again for a double E proc which is pretty good for trading. It also greatly helps his csing under turret since he has low Attack speed, so overall a good change.
TLDR: Darius Q nerf will kill his laning and his teamfighting. Nerfing his HP regen to compensate for the fact he has a heal is a better change and nerfing his W at earlier levels would force him to max it 2nd to reduce his hidden power from E armor pen. Mundo E change is a nice, helps his jungle clear, dueling and csing.