> Your argument basically is a professional motte and bailey fallacy.
No it isn't. A "motte-and-bailey fallacy" is where you have two versions of your position, one which makes broad claims but which is difficult to defend, the other which makes much narrower claims but which is much easier to justify, and you equivocate between them. I'm not doing that.
A "companion-in-the-guilt" argument is different. It is taking an argument against the objectivity of ethics, and then turning it around against something else – knowledge, logic, rationality, mathematics, etc – and then arguing that if you accept it as a valid argument against the objectivity of ethics, then to be consistent and avoid special pleading you must accept as valid some parallel argument against the objectivity of that other thing too.
> And you cannot conclude objectivity by consensus.
But all knowledge is by consensus. Even scientific knowledge is by consensus. There is no way anyone can individually test the validity of every scientific theory. Consensus isn't guaranteed to be correct, but then again almost nothing is – and outside of that narrow range of issues with which we have direct personal experience, we don't have any other choice.
> I argue that you DO need to answer the deep problems in mathematics to prove that 1+1=2, even if it feels objective- that's precisely why Principa Mathematica spent over 100 pages proving that.
Principia Mathematica was (to a significant degree) a dead-end in the history of mathematics. Most practicing mathematicians have rejected PM's type theory in favour of simpler axiomatic systems such as ZF(C). Even many professional type theorists will quibble with some of the details of Whitehead and Russell's type theory, and argue there are superior alternatives. And you are effectively assuming a formalist philosophy of mathematics, which is highly controversial, many reject, and few would consider "proven".