Better use of charged tm: Mamoswine or Tyranitar?

Not a great current need for Ttar against the raid bosses. A double ground Mamoswine or Rhyperior would make an okay addition to your Dialga squad, and longer term a double ice Mamoswine will help with dragons weak to ice.

In my experience, Ttar has more versatility in general and can do a pretty decent job against many opponents, even when in a type disadvantage. It is also harder to evolve due to how many candies it requires, and from what you describe, the candies might not be so readily available to you right now. However, Ttar is also a better rock attacker than it is a dark type one, and you seem to have missed that train (you MIGHT have a chance to get some in December again of they pull off another community weekend for all previous CD species, but that also might not happen). Rock attacks also have a generally better chance to be weather boosted than dark ones, which is a plus.
Mamoswine is the best ice type attacker, presumably for all generations to come, but that is also a very niche spot for it since it only really shines against foes with a double weakness to ice, which in normal gameplay only applies to raiding Dragonite and Rayquaza (which is currently out of raid rotation), and fighting Dnite and Salamence in gyms. It can do a decent job as a ground attacker in some instances, but you can also do well by just using Rhydon or Rhyperior if you happen to own some of those.
It all comes down to your dust levels, I guess. Unlocking secondary moves is a burly decision if you aren't well stacked in dust and I would generally only recommend it to hardcore players cause it can set you back real bad. If you think you can absorb that hit, then the strategy you described seems sound and could get you all set up if you are lucky, but it is a big gamble.
Honestly, if I was in your situation I would wait a little to see if you can gather a bit more resources in general. You do not seem to raid enough, perhaps because of your situation or your local raid scene, so I wouldn't be in a rush to optimize your current options. I would follow the starting strategy for newcomers of building a team of decent weather boosted Eevees into Espeons and use those to farm Machamp raids (dunno if they are currently raidable, but in case they are not, you still need to get the Espeon team first regardless). Then use the Machamp to short-man more Ttar raids and hope you get one with a better moveset. That will also get you more Ttar candies, which is never a bad thing.
All in all, I think you should play the waiting game and be patient, the way this game is designed favors being patient over anything else. Once you get to a point where you have enough battle fodder to throw against gyms and low tier raid bosses you can start to evaluate where to put your hard-earned resources to better use.
If you decide you cannot wait, however, and you are really aching to optimize those two options, then I'd try your plan. I would not evolve the shiny larvitar by no means. It is very low level and makes for a better trophy than a fighter. Remember that IVs have a very minimal impact on gameplay and, in many cases, you are better off just evolving and using the high-level mons rather than tanking thousands of dust and candies towards making low-level, high-IV options up to usable levels, unless you can make that jump in a single power up session.

I've had some bad luck spending charge TMs recently, so I don't use them until I have 4 which is what it took to get from a B move to an A move recently. Spending a TM to end up worse off than I started is not fun.