Hi again, and sorry if i answer to your mail so late.
About your question on how to balance the craftworls and the pirates,
Balance wise ain't bad.
supernova: you suggest?
for now i can't say details, cause i dont know clearly what will be your new rules.
Go by v19 I say.
Speaking of the background:
Pirates should have faster hit and run ships, not like before, when Wraithsips were more agile and costed much less than their pirate counterparts.
Eldar Pirates are what says their name: Raiders, they rarely have a place to defend, they assault transports to survive, and escape when larger fleets come to hunt them. So their ships should be designed for speed and agility to survive.
Craftworld Eldars are different, they are a regular army, a fleet builded to defend their artificals homeworlds, so a fleet that many times can't retreat.
They should have ships heavier and with better armaments.
Corsairs focus on small, easy to hide, ships, Craftworld on cruisers. But both are Eldar and both use the same 'substance' (Wraithbone) to build their ships. Both use sails, both have same weaponry. The difference in design is rather aesthatic without a good base in background.
Speaking of Ship Classes
Don't "retouch" the ships cost if possible, make craftworld pay for his firepower and pirates pay for their agility.
Pirates: Make the pirates cruisers turn 90°, so can act in formation with its escorts (the heavy cruiser and the battleship should still have less manouverability).
You could also give the Eldar heavy escort 2 hull points, after all is huge, and costs so much.
Craftworld: Make Wraithships turn 45°, mantain its firepower and the weapons selection, if compared to the pirate cruisers will have less power & agility, but will cost much less too.
Give to the Dragonship a weapon range of 45, that compensate for its lack* of manouverability and speed (*compared to other eldars ships)
Don't forget that craftworld ships should have access to boarding crafts & torpedoes, and this alone, make their capitals ships "heavier" than the pirates, even without adjusting their cost.
Costs, that is always tricky as 45cm weaponry is a lot on a fast moving Eldar ship. A 90* turn on a Corsair cruiser heavy, in the past MMS did have 90* Eclipses & Shadows. This led to cruiser heavy fleets, a thing I was wanting to avoid. 90* means the ship can do a complete backturn in one phase. That is so hard to play against. Almost reckless play suggesting; and Eldar should never be reckless. So I understand your point but a lot of playtest has seen the turn rate for a balance issue.
Heavy escorts: aha, well... that is already a debate on its own within the BFG universe. As long as there is no official 2 hits escort I will never take one aboard in MMS. Core rule breaking as msm does, ya know

On weapon range. The WS / DS will make it possible to do a 25cm + 25cm + 45cm strike range = 95cm !! That is the longest strike range in the game apart of Nova Cannon which is unreliable. On a Dragonship that would be 3 Pulsars or 12 Batteries doing a 95cm strike range! And staying at a distance of 45cm!
See, the Eldar would do a setup for his fleet, then lockon and blast away with his long range gunnery from Wraithships & Dragonships. The finesse would be gone. It would be too easy. They would not miss the 90* turn.
Yeah, CWE has aspect warriors & vampire raiders.
Fleet Lists Ideas:
Till now i've spoken about single ships balance, but we have to consider the fleet formations too.
Usually the battles are done with fleets of 750 or 1500 points, and many fleet lists are done so you cannot have a Battleship with only 750 points.
Imperial and Chaos, at best, can use a grandcruiser like the Vengeance into battles of 750 points as well.
You could arrange the eldar fleets so, with 750 points the pirates cannot use their supernova grandcruiser, and must play with 2 cruisers and 4 to 6 escorts, or 1 cruiser, 2 light cruiser, and 4 to 6 escorts.
You coul also limit the pirate list so you must have 2 escorts (or 1 heavy escort) for every cruiser, and 1 escort for every Light cruiser.
For larger formation, you could put a limit of 1 supernova for every 1000 points (and 3 escorts).
An Eldar craftworld fleet of 750 points, should be able to play with 2 Wraith and one Dragonship.
A fleet with 1000+ points could use the Flame of Azurian special Dragonship.
Tau, Tyranids, Imperial Navy, Adeptus Mechanicus, Space Marines can all have a Battleship under 750 points... What's left

Limiting the Supernova? As it is a rare ship I can do a limit on it. What do other people say on this?
2 escorts for every cruiser will make a pretty heavy cruiser list.
I think the current lists (apart of the light cruiser note!!) is okay.
I hope i wrote it well enought to be understood.
No worries, all good! Thanks.
Now, about my idea of the holofields, i'm sorry you didn't liked it, but still i dont agree with a thing you said:
Your holofield idea :
I do know some people dislike the fact if lances didn't hit on a 4+ (rule mechanic).
You should dont care too much of what people like or dislike, of course you must consider different opinions, but you are the one who's writing the rules now, so you are the leader.
As a leader you priorize game balance and game fun (fast and easy rules that reflects the racial traits) right?
Every time a rule is changed, of course some formations can loose strenght and some can gain it, but still, as long the overall game is balanced, peoples should not complain.
At this point in MMS, after a lot of versions and improvements I take care a lot of what people think before I ammend a change. The v19 list is pretty well used by the groups who dislike official rules. I cannot do a 'bulldozer' on it

[uqote]Let me explain it with mathematics:
I dont understand how people dislike if can't hit with lances on a 4+, but don't complain when after that, the eldars have a save roll.
In the past you gave a save roll of 5+ on lance's hits, and a lance scored an hit on a 4+.
That means that you hit succeful 4 times on 6 multiplied for 3 times on 6.
The result is 2/6, that is equal of a roll of 5 or 6.
Statistically speaking you already made the lance hit on a roll of 5+ in your mms 1.9 rule mechanics (that was alot better than the 1 on 12 of the original ones).[/quote]
Because the save roll, is an effect that the Imperial Navy or Chaos can aqcuire in a campaign setting. Yes, they can get an upgrade on the shields to have a saving throw (campaign section). So a saving roll exists and it does not break the core rule.
Yes, math is the same, but I should also follow my set doctrine.
Math wise the Leadership test (with mods) can do the same as a saving throw or adjusted dice roll.
hope i've been helpful.
Certainly, I just convinced myself why 45cm weaponry is not cool with the current weapon systems.
cheers!