I agree on 1 and 4 with Helmic.
2. For that specific loadout, you better have a pipe wrench. I can't describe the frustration when I see a random engi running round with a buff and spanner/mallet. Pipe wrench gives you the sacrificial jack of all trades, master of none, but the buff helps you compensate. I can't think of an instance where a mallet or spanner will be good for the entire match full of varying situations.
3. Unless you are in a group like Polaris and can pull off that kind of coordination, than take spyglass. Like Smollett said though, a coordinated team effort can make for interesting gameplay, though because of its layout, the Spire will benefit most by it.
2. As people keep mentioning in the fire thread, it's not incredibly risky having just one engineer with an extinguisher on some ships, particularly the Goldfish. Mallet/spanner/buff allows you to rapidly repair AND buff in between those repairs. A single engineer with that loadout can fight an enemy carronade very well by allowing the captain to rise rapidly without sacrificing balloon HP, or give the hull a bit more HP if they switch out with the main engineer while he goes to combat engine or balloon fires. If there's three engineers I almost expect the buff engineer to go mallet/spanner/hammer, there's just not enough components to catch fire to justify that third extinguisher and there's plenty of reason to repair and rebuild faster than a gunner.
Of course, there's also the theoretical situation where you KNOW there just won't be any fires period and you have your main engineer go mallet/spanner/wrench and the buff engineer go mallet/spanner/hammer. Buff the hull and keep it at 100% using the most efficient tool at the moment.
3. Even with the coordination the benefit seems pretty situational and negligible, it seems like Polaris would have won that match no matter what just by the sheer difference in skill so I'd have to wait to see them pull off something equally insane against the Paddling and win with it to be convinced. Maybe in CP? At least there being down an engineer doesn't mean as much when traveling from point to point, you're not likely to take a lot of damage unexpectedly or need to shoot a gun without plenty of warning.
2. Once again, no. You can't afford to lose the ability to either repair or rebuild quickly. Ever. As an engineer you need to be able to do both.
Except gungineers may get more use out of a mallet if they're never leaving their gun, just as a lot of people taking the gunner role like to bring a mallet over a pipe wrench. In which case they're just using buff and chemspray (to get the effect of charged, heatsink, AND their preferred ammo type at little penalty) in place of two extra ammo types. I'd still advocate the pipe wrench for both gunners and buff gungineers, but it's not quite as black and white as it is with his first question.