Yes, 75% of the House can't attend those meetings. Because they're committee meetings. 75% of the House isn't in those committees; they're in other committees, that have their own work to do. And again, there are Republicans on every committee.
Republicans can call witnesses.
They can't discuss publicly what was discussed in the room because what's being discussed in the room is classified. It's literally the entire reason SCIFs exist.
Members of the committees get briefs. Members of the public do not get transcripts, until they've been redacted to remove classified information. Because national security.
Anyone in any of the committees involved in the impeachment process can attend any hearing.
The President is not on trial. This is an inquiry. Once a trial begins, his lawyers can have their circus.
The President is not on trial. None of the rights you've laid out apply here. They apply once the trial starts.
It's not a report that Congress decides on. It's official articles of impeachment, which are written by the Judiciary Committee following the same rules and procedures used in drafting legislation and then submitted to Congress.
Schiff isn't the only one in the room, and isn't the only one who'll write the articles. If he writes a report, cool, but he doesn't have to, and others are allowed to do the same.
The Nixon impeachment process was WAY more hush-hush. The title of this article isn't helpful, but it was written by a guy who actually worked on the Nixon impeachment inquiry. Worth a read. https://www.nbcnews.com/think/opinion/republicans-absurd-complaints-about-impeachment-inquiry-access-are-historically-ignorant-ncna1071751
Nixon's lawyers were given access to the inquiry, but not because they were required to. It was something that the Special Counsel allowed as a courtesy; Trump has not earned courtesy, and if the tables were turned and Schiff was in the hot seat, the same would happen because Schiff has not earned courtesy from Trump either.
Most of the Nixon testimony was behind closed doors. Again, you are confusing the impeachment inquiry with the impeachment proceedings.
THIS IS NOT A TRIAL. Once articles of impeachment pass, THEN all of this stuff is required, because at that point things actually have consequences for the President.