It’s interesting that Baha’u’llah says this;

Say The first and foremost testimony establishing His truth is His own Self.
Next to this testimony is His Revelation.
For whoso faileth to recognize either the one or the other He hath
established the words He hath revealed as proof of His reality and truth.
Gleanings 105-107

In brief my reading, including questions, is that;

1 The foremost testimony is His own Self. What is that Self other than Baha’u’llah as a man of a particular time and place? The Holy Spirit I suggest.

Is the ‘Self’ that is the foremost testimony perceivable by all or some today? Yes in all times there are a few.

2 The Revelation isn’t the words.

3 Words are for the ‘doubting Thomases or Thomas-inas who can’t ‘geddit’ via one or both of the first two testimonies.

4 Realizing the Revelation is not a matter of reading the words.

5 Proof of His reality and truth can be gained via all three means but perhaps the first is to be preferred?

6 Regardless of how you, or I, came to ‘geddit’ dedication to a combination of all three testimonies is likely to be a good idea!