Ambiguous phrasing
Separately from when the action occurs, I feel the rules are also not clear enough on how the constriction happens, i.e. whether it happens automatically or if it should be considered a separate attack option. Going by RAW the tentacles constrict as part of a successful attack, which is what makes them an unbelievably powerful tool. In addition, the exact wording indicates they do not necessarily have to constrict each time they attack;
Tentacles may constrict foes, inflicting (Strength+1) lethal damage
per turn. - V20 p. 189
That said, to answer your question we need to take a look at the nature of the attack. As the constriction aims to deal damage as well as prevent movement, it is essentially a clinch.
Combat sequence
Starting from your third point and keeping in mind the rules for a clinch, the sequence would look like this;
3.1. The spawned tentacles can each be used to attack enemies within their reach using Dexterity and do damage with Strength + additional successes from their Dex roll.
3.2. In case of a successful hit, the Lasombra may choose to have that particular tentacle constrict the enemy.
4.1. At the start of a constricted enemy's turn, a separate contested Strength check is made against each tentacle. The enemy needs to beat the tentacle's successes to free his limb.
4.2. If the enemy breaks free, he may take any action as per normal. Should he fail to break free of one or more tentacles, he is limited in his actions as per a clinch. Keep in mind that he is most likely still free to bite, claw, head butt or do damage in other ways. In case of a Kindred, biting is most effective since the tentacles cannot soak Aggravated damage.
5. When the Lasombra’s next turn comes up, he can direct any tentacles still constricting the enemy to deal the crushing damage (Strength+1) and any others to attack as per point 3.1.
Sadly, while the rules indicate the Lasombra does not need to concentrate to direct the tentacles, they neglect to state whether this happens at the start or end of his turn.
Swarming is a tactical choice
I'd also take a look at point three as bumping the difficulty would be a case-by-case thing. As you mentioned in point 1, the number of successes determine the number of tentacles to be spawned. But they do not necessarily need to originate in the same location. As long as there are enough shadows within line of sight, they may be tactically placed around the area. This means you lose the advantage of flanking him by introducing multiple 'combatants' within reach, but it can be a tactical choice to control the field when encountering multiple targets.
Updated rules in DAV20
Depending on which rule set you are using, it might be entirely different. Although the flavor text still mentions grappling, the ability to constrict was completely removed from the system text in DAV20. Furthermore, the tentacles must originate within 20ft of the caster and receive a Strength and Dexterity rating equal to his Obtenebration rating without a boost from Potence or Celerity. Tentacles attack using Dexterity and deal Stenght+1 points of bashing damage.
