As I understand it, Bellof had signed a one-year contract with Tyrrell for 1984, with the team having the option to extend the contract for '85, '86 and '87. When Tyrrell decided to exercise this option, Bellof's manager Willi Maurer tried to have the contract declared null and void, as he claimed that Tyrrell had broken the terms of the contract by not providing a car for Bellof in the last three GPs of 1984, following Tyrrell's disqualification from the World Championship. This dispute led to Tyrrell replacing Bellof with Johansson at Rio, but things were sorted out in time for Bellof to return at Estoril.
So if things hadn't been sorted, would we might have seen Bellof at Ferrari instead of Johansson? That is an interesting thought.
In regards to the first question, him going to better team for 85. It looked like all the good rides were taken. Brabham needed someone with money and took Hesnault. Williams picked up Mansell, while Senna was picked up by Lotus. Arrows, I'm sure required both Berger and Boutsen to bring some sponsorship money. And, with the prospect of Tyrell getting Renault Turbo motors, it looked like a good option at the time. Maybe, that answers that question....