That's one hell of a lot of things to go wrong. If you're going to have someone on the inside anyway, which your scenario presupposes, why not just suborn a few members of the crew of the target vessel, then land your commandos as part of an 'exercise'? Your inside guys keep the ship from firing on or evading the helos until they can land, then your heliborne commandos complete the takeover.
Even simpler would be to buy off the ship's officers and get them to sail the ship to a destination of your choice ala 'Hunt for Red October'.
My guess is that most countries WOULD shoot down an aircraft that refuses to ditch alongside and tries to land on deck to be honest.
Probably so, I think, although probably more to stop the spread of smoke from fires than due to serious concern about gas attacks.Even if you could smuggle Sarin or some other toxic gas (why bother with knocking them out when it's just as easy to kill them?) into the ship it wouldn't affect the entire ship; the ship's ventilation is deliberately compartmentalized to prevent vapors of any kind from spreading throughout the ship. AFAIK this is a standard design feature for all USN vessels larger than five thousand tons or so, and probably for most other navies also.
A modernized battleship like USS Iowa (Yes, I know it's retired, this is a theoretical scenario)?
Even if you could smuggle Sarin or some other toxic gas (why bother with knocking them out when it's just as easy to kill them?) .