Is it illegal for the hotel to collect conference fees, issue service invoices and collect taxes from the other party and turn them over? How to deal with accounting?

The hat is a bit big, which is a hidden rule. As long as your account is unclear, it is not illegal. If your hotel doesn't do this, will the conference group come to your place for a meeting, and the income from accommodation and catering will be gone, which is a win-win situation.

How to deal with unconfirmed income in accounting?

Accounting treatment, your hotel should have a lot of income without tickets, just fill it in cash.

Is it necessary to make tax adjustments?

No, as long as the ticket is issued, the income must be confirmed.