Sentry rules questions -

For Marines:
When a Marine uses their Tactical Move they "go into sentry then move etc etc " - using 2 actions. Must they use up the action or do they remain in sentry to use the action point at a later time? Example - Marine used tactical move to to move one tile and remain in sentry to fire,move,cc at a later time? Does this violate the actions use or rules since you must spend an action to go into sentry then use the action to perform a simple action?

Sentry for the other factions don't utilize a tactical move are they able to move, use sentry, then use that sentry token to use a simple action? Move, shoot, cc?

IF a card or other method enables sentry - can the same action be used more then once in a player turn? Example - a marine moves, then shoots a target - then plays a card placing the model in sentry - can that model shoot using the sentry action?

Its a special action costing two actions allowing the Marine to go into sentry then move. If the move brings him into Line of sight he my use the sentry action to fire before the target. otherwise he remains in sentry mode until it is triggered by something coming into line of site.

I would need to read the cards before answering your second question, as the wording on the card may say cannot be used on an activated model etc.