I don't think it is as good as deepseek v3 for updating coding tasks. I use 2.5 pro as architect with deepseek v3 as editor engineer. This combination work almost so perfect. Flash has advantages of great context size though. gpt 4.1 is also great.

Yes. Any API Key is allowed, Also you can assign different LLMs for different modes. It is great for cost-optimization. Like architect, code, ask, debug etc.