As I'm not very involved in tiering process or #littlecup discussions, my opinion may be a little ignorant so I beg your pardon in advance.
I agree that non-speed boost Carvanha and Yanma should be unbanned based on how balanced they would be in the current Meta.
Carvanha's typing is both a blessing (resisting Shadow Ball and Sucker Punch, immunity to Psychic, good offensive coverage) and a curse (weakness to Fighting and Electric). Adding to that is the fact that it cannot secure KO's as easily because of Drain Punch being here and there. To top it all base 20 Def and SpD puts it on the level of "Top Defensive threats" like Abra, Feebas, Caterpie and Magikarp (even Diglett has better defenses when uninvested). In a meta full of Eviolites and Mienfoos and Timburrs and HP Fighting and so on, Carvanha is not going to sweep without heavy investment and team support.
Yanma is a strange 'mon to be theorymoning as it seems more balanced than Carvanha, it even has a good defensive typing 4x resisting Fighting (the defining type in this gen), immunity to ground and good offensive capabilities. However, its 4x weakness is very crippling. It looks like a faster Larvesta IMO. In this metagame, however, this can be decisive as Speed plays a major factor. The thing is that this bug has something other Speed 20s lacks: reliable recovery. When we factor this in we can see a great advantage on Yanma over other threats. Yes, Murkrow has 19 speed and reliable recovery but when Murkrow roosts on a threat that has a Fighting-type move it risks heavy damage because of its secondary Dark-typing. Yanma doesn't risk anything as it won't gain an added weakness (Bug still resists Fighting and also resists Ground). It even looses its 4x weakness and an electric weakness. Even its bad offensive typing is mitigated by Tinted Lens the
de facto ability for offensive sets. Is Stealth Rock and Eviolite enough to stop Yanma for being broken? I think that Speed Boost was not the only thing breaking this guy but I think only time can tell if and when the Council seems appropriate.
In regards of LC banning policies:
Also, I'm not really sure why this is our policy in the first place lol.
I'm not quite sure why banning a whole ability is "harder" than banning a pokemon. I think I'm missing something here but banning an ability is not as complex in my head. Speed Boost has proved to be Broken by itself as a pokemon who was "very good but wasn't banned" in DPP instantly got banned just for its access to this ability.
Finally, I have battled Torchic with Speed Boost prior to PS! inforcing LC legality and while it was annoying a single Thunder Wave user or a hard hitting 'mon on the recipient was enough to stop Speed Boost Baton Pass. Competent players may be skilled enough to capitalize the advantage but Baton Pass is not a very good option as most threats will need a +3 boost to outspeed Drilbur in Sand, a feat that is hard to obtain.
In conclusion, I think we gain a lot of diversity (maybe a rise in usage on long forgotten Steel/Rock) and two interesting additions to LC for as long as gen V is still going. If anything, when they look broken again we can just ban them again as another Gligar-esque situation.
Edit: I will write faster next time... maybe.