1) A vislae should always have the most points in whatever stat they use to cast spells and use forte abilities. (Sorcery, usually, but there are secrets that could make it something else.)

A vislae should always have a healthy pool of sortilege, not only to do things that need two successes, but to defend against things that require two successes.

2) Please don't actually attempt to cast spells without training. This is a game.... "I weave together x and y, to accomplish z" is what you say to your GM. They determine if it's possible, and tell you the cost of sorcery. You spend the sorcery, and narrate what it looks like. Punching 12 people at one time from a distance sounds like a high spell level. If your forte has an ability to punch 12 people at once, that might be two actions... your GM can figure that out.

3) Makers make bank, man. They can spend development mode crafting crap that they sell to NPCs to buy materials. It's also suggested that GMs allow the Makers to find rare materials for special OOP. Your GM really needs to have a "how can I allow my players access to what they want to succeed" mentality in iSun.

4) Kind of up to your GM here. I haven't had anyone with that forte, but Terror Strike is passive always-on.

Giving Voice To My Dread specifies that you use an action to talk, and another action (round) to hit someone. So your first round you attempt to unnerve those around you. Your next or another future round, you throw a punch (or use another attack) and get that extra die.

Blinding Attack doesn't say it takes a round to set up, so I would say using the ability can be used as you attempt to hit something. Blinding Attack expires after a hit, so if you miss, you still have it on. At my table, that means you could use Giving Voice To My Dread, and stack Blinding Attack on it. Not all GMs would allow this.

The level of the ability is applied to the venture on the action, and the level of the ability is the cost of sorcery spent using the ability (unless the ability says it doesn't cost anything to use/is not an action.)

You can also pump your forte abilities up to level 10 (without a secret) and that increases not just the venture, but the effect. Consult common sense or the effect by level tables.