https://www.rollcall.com/news/congress/lots-no-shows-impeachment-inquiry-depositions
https://definitions.uslegal.com/d/due-process/

Sensitive information? I do not agree the fact that every meeting of the Adam Schiff's committee was isolated in secret and denied the presence of the Republic committee.

That is not the only reason it could been held in closed door.

Overall the impeachment is wrong. Nothing to impeach. The president has the right to investigate any diplomatic involvement with foreign countries, under his power, especially when it involves material consideration and mutual diplomacy.

The house has the same responsibility if something is found wrong the house has to investigate.

No sound judgement could has been written in books neither practiced and decided in the Court of Law.

Inpeachment is not criminal case congress can't deal in criminal cases like you stated each branch has it own responsibility.

It was a complete denial of due process.
Criminal
By your statement it sounds like you are referring to the criminal due process
Due Process Law and Legal Definition. ... Due process requirements apply to both criminal and civil law. Due process generally requires fairness in government proceedings. A person is entitled to notice and opportunity to be heard at a hearing when they have life, liberty. or property at stake.

Inpeachment house rules page 23
Due process rights of respondents (p) The committee shall adopt rules
to provide that (1) not less than 10 calendar days before a scheduled vote by an investigative subcommittee on a statement of alleged violation, the subcommittee shall provide the respondent with a copy of the statement of
alleged violation it intends to adopt together with all evidence it intends
to use to prove those charges which it intends to adopt, including docu-
mentary evidence, witness testimony, memoranda of witness interviews, and physical evidence, unless the subcommittee by an affirmative vote of a majority of its members decides to withhold certain evidence in
order to protect a witness; but if such evidence is withheld, the sub-
committee shall inform the respondent that evidence is being withheld
and of the count to which such evidence relates

Ans. Republicans assigned in the committees did not participate because Adam Schiff did not allow them to.
As per Roll call
All 15
Adam Schiff California dem
Jim Jordan Ohio oversight ranking
Mark Meadows NC oversight GOP

Devin Nunes California 8
Elsie Stefanik New York 9
Michael McCaul Texas 7 Rep Foreign Affairs ranking
Eliot L. Engel New York 3 Dem chairman
Scott Perry Pennsylvania 13 Rep Foreign Affairs
Lee Zeldin New York 11 Rep Foreign Affairs
Elijah E. Cummings Maryland Too ill Dem oversight
Carolyn B. Maloney NY 7 Dem acting chair
18 never attended

In total, the transcripts show 14 Republicans and four Democrats did not participate.

The fact that more Republicans did not participate than Democrats is noteworthy because the GOP complained that all members weren’t allowed to attend the depositions, with some even storming the secure area where the proceedings were held in protest.
In addition to Crawford, the Republicans nonparticipants were: Oversight Reps. Paul Gosar of Arizona, Virginia Foxx of North Carolina, James Comer of Kentucky and Mark Green of Tennessee; and Foreign Affairs Reps. Christopher H. Smith of New Jersey, Steve Chabot of Ohio, Joe Wilson of South Carolina, Adam Kinzinger of Illinois, Jim Sensenbrenner of Wisconsin, Ron Wright of Texas and Greg Pence of Indiana.

The only republican asked to leave was Matt Gaetz, Jordan stated that after 20 hours of testimony with only 12 present a 13th shouldn't be a problem.
The removal was finally completed after the ruling from the House parliamentarian. Gaetz agreed to leave the room. These depositions were held on Oct 3rd and 11th, the house was on reccess.

This is the full reason House parliamentarian cited when he requested that Mr Gaetz leave,

The exchange occurred during opening statements and Schiff’s staff had to get a ruling from the House parliamentarian, who determined House deposition regulations have always been construed to only allow members of committees conducting the deposition to participate. Gaetz agreed to leave the room.

Ans. You are talking about a different scenario of the radical dems closed door meeting.

Also if you remember the group entered a area where phones aren't allowed
and since it was supposed to be secured in the first place it would be logical
to have the door locked. The same house rules used by the Republicans to keep Benghazi investigation in a closed door meeting.