Attack is definitely priority one, since you'll be doing normal shots for the majority of the game. Well, when you're not reloading that is. ._.
So you really want to make each shot count. It's really hard to get any awards with the musketeer without landing every shot, let alone any kills, since you spend such a ridiculous amount of time reloading. Adding strength will really help give those precious two shots some punch.
Personally, I wouldn't put any skill points into the rapier skill, if you're trying to fulfill the ranged support role. Unless musketeer is going to be your only perm, and you have nothing else to switch to during close combat, you're not going to use the skill much. I find that it's great for comboing into a grim air combo, but that's only if you have a grim of course. Also, melee combat with the musketeer is just plain awful. It's unnecessarily more difficult than usual, since the hat skill pushes your view back so much. Basically, switch to another character if you're not sniping with the musketeer. For that reason, I probably wouldn't advise adding to defense either.
The weapon skill, by itself, is a pretty lackluster skill (compared to other weapon skills), but it's great for what the musketeer is and needs. Essentially, it functions as a guard breaker shot for the musketeer, and adds an extra shot to his ammo count. The KB on the skill is decent, at least better than what normal shots offer. The problem is that the skill charges much too slowly for what it's supposed to do.
So taking that into consideration, whether you'll opt to add to the weapon skill or not will depend on how you want to use your musketeer with your other characters. Are you sure you would rather use musketeer's weapon skill over your mage's skill (meteor / blizzard?). You'll have to play with the musketeer a bit more before you can really answer that honestly.
For example, I always considered doing full spec weapon skill, but after I actually used the musketeer with my ice mage, I find it much more effective to simply combo the cannon with blizzard for guaranteed (potentially double) freeze, and use the recharge time shooting off normal shots.
Also, I know that you said that your weapon skill idea was based on the notion of engaging in ranged battle. Since you're using arrow toss and grenade (both of which are hard to use against an experienced 1v1 opponent), I'm assuming 'ranged battle' refers to those campfests that always happen in Wild West. .-.
For that purpose, cannon works better than grenade, since it provides two bursts of AoE splash damage, rather than one. Not to mention that since it's not an instant activation skill like the grenade is, it's much less noticeable and predictable.
Don't rush to use up all your skill points though, since you might regret it later if after you've obtained a better grasp of how you use each class in conjugation with the musketeer. Although from my experience, regardless of what skill you want, I'm definitely an advocate of high strength for the musketeer.