I don't have much in play-testing to offer, as I don't actually play harlequins, but please take my advice, as I am interested in them and I have played 40k all of 3rd edition (so Im quite experienced in this new addition, and the armies of it)
Alright:
Dismount rules are awesome, why they never had em before boggles me. Flipbelt rules are new and have problems you will see with jetbikes.
Mime disruption is... odd... but because I have advice for your mimes as well, I shall leave disruption until then.
The weapon additions on the jetbike are nice, but your new flipbelt rules present a problem with a lack of saves for models with jetbikes in cc. I'll deal with it when we come to the jetbike squad.
The stormgolve is the replacement for riveblades I guess? I like the name riveblades better personally.
The main issue with your wargear I have is the flight belt. The flight belt is too much of a compromise between the phase field and the jetbike. Why take a phase field when you have more assured (and further) movement with the flight belt. I think that the flight belt should be dropped (again). It seems that the flightbelt was added just to add another FA choice to the harlies, which I will deal with later. The phase field is much more 'unique' than the flightbelt, so it should stay.
I think that familiar groups should be armed with S.pistols with bio ammo, not fusion.
You should not be able to 'move' the webway exit each turn, thats silly, you are already giving them something that can be arguably better than a waygate anyways.
In the last laugh, does the model attack back with just a normal attack, or does it use its special attacks if available? (does a power weapon model who dies strike with a power weapon attack?) If thats the case, you may want to make it say 10 points more, Im not sure...
The laughing god masque should keep deamons summoning to 12 inches, so its actually useful, but might need to cost more (its impossible to judge masque costs without testing)
Red death is like withdraw right? Sweet
Shifting shadows is IMO, the ones harlequins need the most, and the most thought out one, good job.
The IoY spell needs a range, me thinks, to stop possible abuses of it.
Master mime needs to cost more points, as he is better than a callidus assassin, who has similar rules. Im thinking maybe 85-95 points... at least. (master infiltrator is a godly power)
The warlock group seems... odd and out of place. Just another group of followers in the army. The powers are too simple, and the rest of the squad too much like other harlequins, dump them.
Eldar allies should be at 0-1(to prevent abuse of other armies choices, and keeping the army still mostly harlequins), and maybe not allowing aspect warriors. Aspect warriors have chosen a path, and probably wont just leave it to go join another band. Guardians, rangers and basic DE units make sense though.
Mimes... I am not fond with the mimes you have put up. Frankly, finding a harlequin that can pass the ritual is hard enough. I think they will give them friggen holosuits so the dont just die soon after they join. Mimes as you have them are probably one of the easiest units to kill, period, we can't have that... Since they are new to the masque, make them have lower stats, but the same wargear. Give them WS - 4 and A - 1, to represent their lack of training. Give them flip belts and holosuits though, harlequins are screwed without them. Make them cost maybe 15-18 points?
Jetbike squads do not get 4++ saves in cc with your rules. This is... odd. I advise you have a special rule thats is throughout the ENTIRE harlequin army (except maybe mimes if you are feeling cruel). If I did this rule, I would give the ENTIRE army a 5++ save in close combat (yeah, wyches get a 4++, but they are hyped on futuristic speed). Independent characters (HQ/Elite characters, and DJs) in the army however should recieve 4++ cc saves because of their extra experience/training whatever. How about that?
I propose that your FA venom should be more heavily armed than your transport version, as it is a full choice. How about...
Asp - 55 points
Armor: F: 10 S: 10 B: 10 Ballistic Skill: 3
Armed with a shrieker cannon and TL shuriken cats. Can upgrade shrieker cannon to a shuriken cannon at +5 points, or a bright lance at +15 points, or an EML at +20.
Can have a holofield for +20 points
Transport: The Asp can transport a single Independent character
Somehow the idea of a skimmer flying by and a harlequin jumping it for a quick ride seems sweet to me.
Um... yay DJs are heavy support... that makes so much more sense.
The idea of a looted vehicle seems odd to a 3rd editioner like me, who thinks of us Eldar as total elitists, but what if...
I think you should only be able to take other races skimmer type vehicles. The looted vehicles need to be able to keep up with the normal vehicles, after all. Plus, you dont want ever freaking harlie player from here to london using a land raider with a holofield, now do we?
*rests hands* Thats all, I think.