In short, "arcane spellcaster" and "can cast arcane spells" are functionally identical.
In order to be considered an arcane spellcaster, you must be able to cast arcane spells. Likewise, if you can cast arcane spells, you are an arcane spellcaster. There were rules in early 3.5 that classified creatures with spell-like abilities as arcane for certain purposes, but that was errata'd out.
I’m going to answer “how do I combine Dervish with Lightning Maces,” rather than specifically how to get a slashing mace.
Aptitude Special Ability
The aptitude special ability from Tome of Battle can be applied to a weapon to cause feats that are specifically for another weapon to apply to the weapon with aptitude. It is likely that the designers meant just to let you switch Weapon Focus (dagger) to your aptitude longsword and similar, but the wording of the feat allows even feats where you never had a choice about the weapon type to apply to the aptitude weapon. This is frequently quite powerful, and occasionally completely nonsensical.
If you really want to use maces specifically, you could have an aptitude light mace, and then apply the Versatile Unarmed Strike feat to it. This is probably going to fall into the latter category for most groups, but it’s RAW-legal.
Alternatively, you could use an aptitude slashing weapon, ideally one with a large threat range (the kukri is almost certainly your best bet here: light slashing weapon with a large threat range), and then let Lightning Maces apply to it.
In either case, the Roundabout Kick feat works similarly to Lightning Maces, but for unarmed strikes: once again, aptitude can allow you to take the extra attack with your mace or kukri.
Combined with Disciple of Dispater, the kukris are looking at enormous threat ranges, which means you’ll score a critical on very-nearly every single attack that successfully hits. The maces are only somewhat smaller. Combined with Lightning Maces and Roundabout Kick, every critical triggers two attacks. Your number of attacks is thus more likely to increase rather than decrease; if you hit on the first two or three, you are statistically unlikely to stop attacking until the target is dead.
This is, of course, broken.
Were I your DM, I would allow you to have your slashing mace or allow Dervish to use non-slashing weapons without a second thought. On the other hand, Lightning Maces already is on my banlist. I also consider any combination of aptitude with a feat that couldn’t normally select the weapon in question to be something to be adjudicated by me, on a case-by-case basis.
Best Answer
There is no such thing as "active feats" in the rules
In theory, you can declare to use as many feats as you like at the same time, provided they are compatible with each other.
A simple example of incompatible feats would be Rapid Shot and Power Attack. The former applies to ranged attacks, the latter to melee attacks, so you can't (normally) use both at the same time.
Other combinations of feats are far less obviously incompatible. Let's take a look at Power Attack, Vital Strike and Greater Cleave, since you mentioned it in your question. Power attack simply requires making a melee attack. Vital Strike and (Greater) Cleave are problematic however. The feat descriptions state
and
respectively. Vital Strike requires you to use the Attack action, which is a type of Standard action that involves making a melee attack. (Greater) Cleave is another Standard action type altogether, meaning you can't use both at the same time. Assuming you are using a melee weapon, you can combine either with Power Attack though.