Should I only hunt in spring

Looking at the code, the skills' information is as follows:

• Dabbling - No XP required - No bonus
• Novice - 100 XP required - 1.25% Bonus
• Adequate - 500 XP required - 2.5% Bonus
• Competent - 1200 XP required - 4.5% Bonus
• Skilled - 2500 XP required - 7.5% Bonus
• Proficient - 5000 XP required - 12.5% Bonus
• Master - 9000 XP required - 18.75% Bonus

Thus, a kitten who is splitting their time up between two jobs about evenly is going to still be under proficient when a kitten who'd focused on the same job all the time has made master. You're probably better off letting a small number of kittens focus on Farming continuously through the year; you won't get much benefit right away, but when they finally do make master, they'll be able to feed more kittens, especially with the upgrade later on that doubles how much bonus all skills give.

Even worse, any kitten who has skill in a job they're not currently doing, will slowly (very slowly) lose experience in that job. The effect of this is minor (five thousandths of an XP per second), but if you're idling for long stretches, it can stall progress if you're trying to teach a kitten two jobs.

In the end, though, it doesn't matter too much what you do this early in the game. Getting to master-level kittens takes a while, and swapping your kittens around won't stop them from making master in their jobs eventually; just slows it a little. Plus, once you get more Pastures, and the Hoe upgrades to your farmers, even a small handful of Dabbling farmers will be enough to feed a fairly large population through cold winters.

Each observatory increases the chance to auto-observe, but the chance will never be 100% because it diminishes after 75%.

The relevant part of the code is the following (inside calender.js):

var autoChance = this.game.bld.getEffect("starAutoSuccessChance");  //in %
            var rand = this.game.rand(100);

            if(
                (this.game.ironWill && (self.game.rand(100) <= 25)) ||
                (rand <= autoChance)
            ){  
                dojo.hitch(this, this.observeHandler)({}, true);
            }

Every observatory increases your starAutoSuccesChance with 1, so 11 in your case. However, this.game.bld.getEffect does the hyperbolic effect which makes sure the chance does not become 100%. In another answer(which deserves more upvotes imo) this is perfectly explained, so I will not do it again.

Here you also see is that the chance is compared with a random number under 100 (rand <= autoChance) so the autoChance (or starAutoSuccessChance untill 75 observatories) is indeed the percentage of auto-observes.