A vote for third party is a vote for Harris. You have to hold your nose and vote Trump!
But seriously, Libertarians are the largest third party by quite a margin. So third parties actually help Democrats. It is just the Democrats mentality that they are owed votes for not being Republicans, rather than a candidate’s or party’s job to try to appeal to voters that this narrative is pushed forward.
No, at no point did the Centre try to form a coalition with the KPD, but were turned down. In the Weimar system, it is the Chancellor that is in charge of forming coalitions, so even if the KPD, SPD, and Centre had enough seats to form a majority (which they didn’t), they couldn’t just form a coalition. This is why Franz Von Papen was appointed by Hindenburg, since he was expected to be able to convince the Centre party and Nazis to form a coalition with the conservatives and monarchists. And why when that failed and there was a failure to form a ruling coalition that Hindenburg appointed Hitler as chancellor to create a Nazi lead coalition.