@Panino
They cannot learn from her because everything she did has been leagues beyond them. In their eyes, what she's doing is witchcraft. It's like an outer space alien bringing down an intergalactic ship down into medieval society. The medieval people won't even know where to start researching it. At least if she is allowed to continue at her own pace, she will teach them as they are now her people. How much cooperation do you think a jailed person would give?
Actually, the muskets were not reverse engineered. Story goes that the first musket was stolen from a marooned Portuguese conquistador and a blacksmith married his daughter to the Portuguese man to learn how to make them. Subsequent muskets were bought from the Portuguese at high price before the Sakai merchants were able to make their own passable designs. At this point, they still have to import gunpowder from the Portuguese. Later in the Sengoku period, the warlords were able to make their own gunpowder, but that's years in the future.
What prevents them from repeating what Shizuko did was the lack of understanding of what she was doing. Farming is not really straightforward. If your pH is off, your crop can get sick. If you miss the timing for planting, your crop will not produce good yield. If you don't watch the weather, your crop will either die or become stunted. They can do the exact same thing as she did, but the minor differences in weather can totally wreck their harvest.
You may scoff at honour because you live in the modern world, but to the samurai, honour is worth more than their own parents. As such it is not uncommon for samurai to behead their own fathers if their own fathers betray their lord. Honour was their single driving force, more than gold. That's one of the reasons why ronins were looked down on, as without a master, they were without honour and therefore, were no different than stray dogs.
Also, Nobunaga's followers already knew of Shizuko and where she was as shown in the previous chapters. The only reason why they're not attacking her is because Nobunaga personally put her under his care and placed Mori as her direct superior. If anyone has any problem with her, they will contact Mori, not her.