Arg! too much to reply too!
I'm pretty happy with the defender going to 45cm batteries, it gives it a single purpose then and is a bit different from other escorts too

With the protector i'm in the same boat as Horizon, it's a step in the right direction, but 90* turns would really add to the ship. It is pretty much a upgunned dauntless afterall.
As for the Warden, a 2-IC ship with nothing else sounds and looks cool but would be complete junk against Eldar. What’s wrong with making the Warden a faster Orca, or maybe even a Tau Cobra?
Does it have to be good against every other fleet? The Gothic class cruiser is pretty rubbish against eldar.. do we need to change that too? [/devil's advocate]
It's a bit unique and powerful, but not overly so due to restrictions
If you do take 5, then you must also take neutered emissary to be allowed to take them. Yes you are getting 10 lances for 150. But you are also getting a 2Wb 1 LB 3Torp Cruiser for 130 (using horizon's example)
Which gives us... 10 lances 2 Wb, 1 Lb, 3 Torps for 280. I can get 7 Lances 14 weapons batteries for the same price in an IN fleet (which is approx 10 lances 5 WB.. not that far off really!). Still don't see it being a massive problem, however play testing is the way to find out

(as i'm aware that you could just spam emissaries and wardens... see later about restricting emissaries...) What about if they had 4+ armour??
Restricting the protectors to 2/500 (1/250) is fine by me... That's pretty much not a restriction anyway. My 750 fleet is 2-3 protectors and custodians to make up (usually 2 Protectors 8 Castellans), And one of My 1500 is similarly built, 1Custodian, 2 Protectors, 1 Emissary, 5 Wardens, Castellans for the rest. (I play very fluffily)
From your background/fluff point of view however the ships that should be restricted are the diplomacy vessels. I don't see a vast number of those being used in fleet engagements, nor would there be much point in having more than 1 or 2 in a fleet. I've already commented on the hunting pairs of protectors. I completely agree with you that you shouldn't have fleets of only protectors... but similarly, you shouldn't have fleets of only cruisers and battleships for the IN,.. Chaos... etc. Escorts should be a must in every fleet, and more common than cruisers to boot!

Just a side note... The 4 cruiser class scenario would be 2 Protectors, 2 Emissaries with a limit on protectors. Is this compatible with your fluff/gameplay/fairness measuring? Would 1 Protector per 250 or part there of be a better answer? Would allow for 3 Protectors, 1 Emissary in the Cruiser Clash.. Bit more fluffy etc