Yes, Telegram start parameters support a maximum of 64 alphanumeric characters (and underscores).
No. Typical URL encoding (e.g., %20) is not supported inside the Telegram start payload natively.
Standard `t.me/username` links work for people, but the `?start=` parameter logic requires an automated bot endpoint.
Yes, the t.me server will efficiently proxy the user to Telegram Web or prompt them to open the Native Desktop client.