I find the usefulness of the cloaking device varies depending on the terrain. In wide open areas, such as the outside area around a crashed UFO, it's very useful to keep you concealed while you snipe at the enemy from far away. Best when combined with smoke grenades for maximum effect.
In close quarter combat, such as inside the UFO, you might be better off with a second firearm.
Your agent's primary choice in weapon is also a big consideration. If your agent uses a toxigun only, then it's a good idea to always use a cloak in the other hand, since you don't really benefit from a second toxigun. Other weapons like the Devestator require some thought - as you have a choice between stealth or more firepower.
1.does holding the cloaking device in your hand and holding a two handed weapon give you the accuracy penalty
Yes, all 2-handed weapon will be affected if the other hand is holding something else. The only exception are the missile weapons, since they ignore your agent's firing accuracy.
2. the cloak drops if you get shoot. and if you get shot more it stays off
Ok how long does it drop for 3 secs, 5 secs 10 sec
3. the cloak drops if you shoot Ok how long does it drop for 3 secs, 5 secs 10 sec
I am no expert on how the cloaking device really works, so someone more knowledgeable please correct me if I am mistaken.
I think there are two separate mechanics involved.
The first is with the cloaking device itself. I all it does is reduce maximum distance that an alien needs to be in order to be able to 'see' you. Say if it normally can see you at a distance of 20 tiles, holding the cloak may reduce it to 16 tiles. Sort of like a portable smoke screen. Those numbers are made up for this example. I do not know the exact values.
The second, which occurs even when you aren't using cloaks: When either you or the aliens are hit, you always become visible temporarily to the other side for a very brief moment. You can see this in effect when fighting in smoke clouds. It is hard to guess how the other AI units other than the one that made your agent visible will react.
I haven't counted how long you stay visible, but I suspect it is whatever interval the game needs to perform its regular updates like fire/gas damage, shield recharge, etc. If either unit keeps getting hit, the count resets.