Why is my pastry blistering in the microwave?
This is because you are using a microwave. In theory, if you could turn off the microwaves in the microwave oven, you could use the convection function to bake things. In practice, we have had several questions which indicate that this is not how convection microwaves work. They keep nuking your dough, making it inedible. In your case, they are cooking all the moisture locally, resulting in mini-steam-explosions which create blisters, and a dried out, hard and crispy dough. 

I am sorry, but there is no way you can bake with a microwave, not short pastry, and not other things. One exception might be quickly eaten "microwave cupcakes" which don't have much flour to start with and are baked for a very short time. I have seen people claim that they work, but never actually tried them. 

For baking, you need a non-microwaving oven. The cheapest solution is a toaster oven with a 30x30 cm inner size, they are somewhat trickier to use than a big oven, but good enough for a small budget.