Bonding seacocks is controversial. There are arguments both ways. The point is that many boats have survived quite well without the bonding. So even if your bonding failed, it does not necessarily mean damage to the seacocks.
The traditional way to check the seacocks is, WITH THE BOAT OUT OF THE WATER, take a hammer and give them a good whack. If they shatter, they need to be replaced! If not they are probably good for another year.
About seacocks on above the water fittings. You want the seacock. This is a case where a gate valve is acceptable. My boat (center cockpit) has the
exhaust coming out almost midships and a couple of feet above the water. It has a gate valve that is operated once a year for winter storage. It keeps bugs, small rodents, etc out! Also, the
exhaust can be buried in rough conditions. If there is a problem with the
exhaust system you might want to be able to close it off.