How would Mamoswine impact the Attacker & Defender Tier Lists?

I haven't done any testing personally with my mamo vs dragonite, but there's one real life practical question: How much time does it actually take to beat the defender, accounting for weaknesses/moves etc.

Gyarados is one of the best defenders only held back by the electric weakness. However, even after the 1.6x multiplier change, raikou still cannot kill Gyarados with the first wild charge causing it to perform not as well due to the high EPS but low DPS fast move. Zapdos/Deoxys N (slightly better EPS) or Electivire may solve this somewhat but I still don't think they can take out gyarados with the first charge move, negating any advantage you may have compared to a Rock attacker for example.

This sounds very similar to Mamo vs Dragonite. Mamo 'only' has a high EPS fast move so if it takes 2 avalanches to down dragonite, will it be faster than Rayquaza anyway?

Ice is an amazing offensive type, but a poor defensive one. I think a case can be made for it to be a generalist (in a similar fashion to Kyogre) but it will be a slower, less bulky, but more flexible (2 bar) one.

Any Ice Type with decent attack power or any Dragon, Fairy or Rock generalist and any Metagross not holding Earthquake as its sole charge move will rip Dragonite apart. It does not need to go any lower.

I don't think it matters all that much, rankings are just a by-product of general performance of every Pokemon. Yeah it changes, like you mentioned, but assigning hard values seems pointless when there are so many variables.

The fact that it is weak to Machamp makes it a poor defender (or at least a negligible defender) in my book. Most of us still use a team of 6 machamps to clear out a gym no matter what's in it.

Gym defense: Mediocre tankiness + weak to Machamp = worthless. Use Rhyperior if you want something that double resists Raikou, it's actually very tanky, unlike Mamoswine.

As an attacker Mamoswine is definitely high tier. It has a lot of use cases for past and future legendaries and frequently appears close to the top DPS even when it's not supreme.