Ok, I'm going to start off this reply by being somewhat unconstructive.
I vehemently disagree with all of your assumptions.
* Ranged is NOT more powerful than Melee. It is not broken
* bows vs powder is NOT broken either.
Let's start with Ranged vs Melee …
- In general, melee attack actions offer more + damage options.
- Most ranged actions suffer +1 difficulty die when used while engaged.
- A person equipped with a ranged weapon cannot BLOCK nor PARRY. Two very important defensive abilities.
- WS not only helps a character attack, but WS also helps them PARRY. BS only helps when attacking.
- Melee weapons do not require nor run out of ammunition
Those are just things off the top of my head. If you have NPCs getting killed by bow-wielding PCs before the NPCs can get into melee with the archer, then you are doing something wrong. There are plenty of ways to deal with this. Add more enemies. Have the NPCs use their A/C/E dice to make the ranged attack miss. Make the NPCs suffer wounds (or fatigue/stress) to simply move into engaged range. Have the NPCs hide behind cover so that the PC archers cannot shoot them. Have it rain/fog/snow.etc, and apply extra misfortune (or difficulty) dice for ranged attacks due to poor visibility. At night, or underground, PCs can only see (and therefore shoot) as far as the light source allows them. This might be only Close range. Lots of detriments, and ways to handle, to ranged attacks, without applying a bunch of unneccessary house rules.
Now, Bow vs Powder …
Yes, I will state up front that if you are just looking at combat stats of bows vs powder weapons, bows essentially are slightly better.
Simply put, though, mechanically (and realistically) a bow *is* superior to a black powder weapon. About the only thing 'better' about black powder weapons is the ability to penetrate armor. Remember, these are (relatively) primitive black powder weapons. They aren't modern guns. There isn't Rifling or cartridges, etc. Bullets aren't exploding, tipped, or spinning and causing more damage. They are simply being pushed out of a warped and crummy barrel. In the real world, guns became more commonplace simply because it was easier to train someone to shoot a gun accurately than it was to train them to shoot a bow accurately, and also that eventually making lead shot was cheaper and faster than making arrows.
Lastly, and I like to point this out since many people simply don't realize or understand it. Black Powder weapons in WFRP are symbols of FEAR and POWER. Generally, only elite Imperial troops or the very wealthy (ie nobles) carry black powder weapons and can afford to actually use one. Shot and powder are not cheap, nor easy to acquire. Bows can be found in nearly every town and village, and in every farmer's house. Many people know the rudiments of crafting some sort of bow and arrows, and at the very least there is likely one person in every town of village who does. Black Powder weapons are crafted by specially trained engineers, in very few places. Most folks have never seen nor heard a black powder weapon being fired. It is a 'shock and awe' type of weapon. Merely carrying a pistol in your waistband indicates that you are a person of means and/or power, even without it being used at all. Some will want to rob you, most will fear you (if they recognize it as a pistol, of course). You will gain instant attention from nobles, whether you are viewed with suspicion or respect will depend of course, but you will not be regarded with the indifference they show most Brass Tier people. Essentially, carrying a black powder weapon can bring your effective social tier up temporarily. Then, we can also include the potential firing of a black powder weapon to be Fear-causing in those who are not acquainted well. It can cause animals to stampede or flee. It can deafen people. The powder can be used for all sorts of creative ideas as well.
To wrap up, you cannot judge the benefits of bows vs powder simply based on DR and CR and the flat stats from a simple straightforward combat situation. Black Powder weapons have a host of less tangible benefits to them.
Now, to trying to provide assistance for any house rules. I'd think simply that:
Ranged vs Melee = Require ranged attacks to:
a) Not allow any maneuvers if using a ranged attack action
b) Maneuvers must occur AFTER any ranged attack actions happen.
Bows vs Black powder:
a) Simply add +1 CR and +1 pierce to black powder weapons. They will have better piercing and are more easily able to generate criticals than bows.
Neither of those should dramatically alter the game or rules, but should adjust things a decent amount towards the results that you are looking for.