what do you mean by casting Mark of the Betrayer on yourself? I don't understand.

that confusion aside, i disagree with alot of your assessments. Maim is a passive effect so it costs no mana and has no cooldown, this is a gigantic difference compared to Mark of the Betrayer. the two abilities function differently. comparing them as if they're for the same purpose seems off-base. its true that Regulus has lots of slowing abilities, but only one of them costs no mana and has no cooldown. 

Deadeye I agree is a bad, but Snipe is not. The issue with Deadeye is not that taking Snipe is a bad choice, many Regulus players are extremely effective with this skill in matches of all sizes. The issue with Deadeye is that it has almost no relationship to what Snipe really does (it grants a passive ability to your regular attacks, not a special effect to your Snipe). It does however provide Regulus with his only interrupt, even if its a somewhat unreliable and bad one. 

i think your distinctions between "main fighter" and "support" are not on target at all. both roles are available to each DG and each DG performs each of these roles differently. the choices you are forced to make regarding favor items and gold spending are crucial elements of the game and have no bearing on any particular demigod. i think you're a bit over-valueing Swift Anklet if you see yourself as fatally flawed if you chose a different favor item instead to boost your damage or mana or something else. every DG has to make these choices and you can do just fine without a Swift Anklet. 

i'm still not following what you're talking about with casting Mark of the Betrayer on yourself.

wouldn't that just hurt yourself? it would slow you by alot for 5 seconds and then it would deal damage to you next time you used an activated ability. 

i can't imagine thats what you have in mind. you seemed to be getting at some sort of debuff cleansing effect or something like that. not sure i understand. also, debuff cleansing would be wildly out of place on an Assassin type DG. that's the type of skill that Generals get. 

other than that i agree that Reg's level 15 skills are kinda bad. actually, I like Impedance Bolts quite a bit and I don't think maxing Maim is a bad thing either, so that one's fine in my opinion. the rest of them are trash though and should be changed. 

Not applying the debuff to another demigod but just negating it. I'm running from a UB and he is ready to spit, I quickly cast MotB on myself and continue to run. When the spit hits me it negated due to the MotB.

It takes under 4 minutes for a trio of Regulus's to take down an enemy citadel from max snipe range (which is behind their own tower lines on most maps, and next to the citadel on a few).  That's not good.

As for buffs - almost all of his skills are worth taking, with the exception of vengeance.  And actually, vengeance isn't terrible when you consider the mana cost.  It just isn't worth sticking a scarce skill point in.  It might be better as a damage over time aura, though.  That'd fit nicely with the concept of the Armor of Vengeance.

I've found that Regulus is practically useless without mines early game, he doesn't have the survivability to cap flags or the aoe to farm creeps without them.

Mark of the Betrayer really doesn't need any help.  With that and mines, I see slows that look like around 90%(I've heard it's bugged?).  Then again, in some cases, having it go off before the ability would actually be a nerf.  Sedna could cast heal on herself and eat the damage.  Oak's shield would dispel the slow down.  QoT's shield wouldn't take the extra hit so it would stay up longer.  It's a buff against erebus' mist, he'd have to take the damage before misting, though really Erebus players would probably just bite then mist in that scenario.

With Maim, Mark, and Mines, Regulus already has a ridiculous number of movespeed altering moves.

Giving him speed increase in Angelic Fury, which is very easy to run infinitely even barely mid game, would be a huge mistake, imo.

Actually, a 1% increase per skill selection in AF would be nice. Come on, Regulus himself said he'd move faster if only he had wings 

I agree with the over usage of movement altering skills. If anything needed a nerf, I'd say that was it.

IMO, mines are fine because you have to step on them for one. Second, there are the goggles and the 50gp scroll item that allows you to see the mines. Third, if your UB, doesn't capping out frenzy negate any movement altering skill (I know you have to already be using the skill)?  Minions and creeps can set them off. erebus can teleport. Qot can shield...

vengeance is worthless because of the 2 sec shutdown of skills. By the time you get vengeance, you should already be shooting faster and outdamaging Vengeance anyway (mines even do a better job). If vengeance did a 800-1000 damage then maybe that would make up for the skill point. I'd take the 80mana loss for more damage, than the 0 mana loss, 2 sec cooldown, and 500 damage.

If I max out maim III at lvl 8, I get a 10% decrease in movement speed for 3 sec. If I pick MotB at lvl 3 I already get a 20% decrease in movement for 5 secs and 400 area damage. One is passive, the other does 400, 600, or 800 damage and can either be a 20%, 40%, or 60% decrease in movement.

Remember, Reg has no stun (deadeye is too unreliable) to prevent someone from drinking a potion. Nor does it have a skill to increase your cooldown times. If you drink a potion, more time than not, the Reg is out of mana and fleeing.

If you nerf anything, its the decreased movement speed and fixing the stacking bug.  There just isn't really a character build up with these lvl 15 skills. No fluidity.