Can you Hidden Blade your unit twice? Can you Deathgrip your unit twice?

It depends on what you mean by "twice."

If you mean playing two separate Hidden Blade spells targeting the same unit: Yes, you can play both (each from its own hidden spot), but only the first to resolve will succeed. Once that Hidden Blade kills the unit, the second Hidden Blade loses its target — the unit no longer exists at the battlefield, so both the kill and the draw effect fail for the second one (FAQ #3700).

If you mean using Repeat on a single Hidden Blade targeting the same unit both times: Same outcome. The first execution kills the unit and its controller draws 2. The second execution then can't find the unit on the battlefield, so it does nothing — no kill, no draw (FAQ #11181).

The one exception in both scenarios: if the unit is saved by a replacement effect like Guardian Angel or Zhonya's Hourglass during the first kill, the unit stays on the battlefield as a valid target, and the second effect resolves normally — you'd draw 2 from each execution.

I noticed multiple unrelated questions. I've answered the first one — please ask the others in separate messages so I can give each one proper attention.