It sounds like your pastry dough was overworked, so that the butter was too incorporated.
To get the flakiness you desire, when the dough is rolled flat, there should be layers of whole butter sandwiching floury layers.
If the butter is worked too far into the dough, instead you will get a sandy or crumbly texture.
If you're looking to parboil, likely your best bet is one that you've already dismissed -- inedible casings that you'd remove after cooking. You might even be able to get away with clingfilm, parchment paper or non-stick aluminium foil.
If you really want an edible casing, they do exist, just enter 'vegetarian sausage casing' into your preferred internet search engine, and you should find them. (I assume they're all reselling the same product, as the only option I've found is for 15/16", ~10lb strand)
As you also mention that part of your issue is not wanting to eat intestines, there are also collagen casings which are more readily available, but aren't vegetarian.
update:
To deal with the issue of packing the sausage so you don't have air bubbles :
Place a line of meat down the center of what you're using, roll it up tightly, then seal the ends.
To get the meat lined up tightly: fold the back end of the sheet over the front, hold down the edges and use the edge of a sheet pan to press the meat tightly towards the back. If you're using something that might tear easily (eg, aluminum foil), you can work on top of another sheet of paper (waxed, butcher, freezer, etc.) and fold it over before pushing back, but don't roll it up to make the final sausage.
Best Answer
The best substitute in general for lard would be a combination of vegetable shortening - which is generally hydrogenated palm oil - and butter.
Since this is a vegan substitute, you'll have to just stick to the shortening. That's OK - shortening was quite literally invented to replace lard - but you'll lose a lot of the flakiness and full flavour if you don't mix it with some butter.
For pastry crusts, you might also mix some coconut oil in with the shortening; the flaky texture essentially comes from saturated fats, for which coconut oil ranks pretty highly. Just don't use too much, as it imparts a not-so-subtle flavour of its own.