kare ga kuru to kakusinsimasita.

Often, yes. If the subject is obvious from context, Japanese frequently omits it, so 来ると確信しました could sound more natural in many situations.

However, 彼 is included when the speaker wants to make it clear that it is he who is coming, or when the context does not already make that obvious.

が marks 彼 as the subject of 来る.

In 彼が来ると確信しました, the embedded statement is 彼が来る = he will come / he comes. The が belongs to that inner clause.

If you used は instead, 彼は来ると確信しました, it could still be possible in some contexts, but it would give 彼 more of a topic or contrastive feeling, such as as for him or he, at least. In this sentence, が is the more straightforward choice.

Because 来る is inside a quoted/thought content clause before と.

The sentence is not saying he came. It is saying that the speaker was convinced of the proposition he will come / he comes. So the tense of 来る reflects the content of that belief, not the tense of 確信しました.

This is very common in Japanese:

• 彼が来ると思いました = I thought he would come / was coming
• 彼が来ると信じていました = I believed he would come

So the past tense applies to the act of being convinced, not necessarily to the coming itself.

Here, と marks the content of a thought, belief, conviction, statement, or realization.

So 彼が来ると確信しました means the speaker was convinced that he would come.

This use of と is very common with verbs like:

• 思う = think
• 言う = say
• 知る = know
• 信じる = believe
• 確信する = be convinced

You can think of it as linking the clause 彼が来る to the verb 確信しました.

It is the same particle in form, but not the same function.

Japanese と has several uses. In this sentence, it is the quotation/content marker, not the conditional.

• 彼が来ると確信しました = I was convinced that he would come
• 彼が来ると、始めます = When he comes, we will begin

So here, do not read it as if or when.

Yes. 確信しました is much stronger than 思いました.

• 思いました = I thought / I believed
• 確信しました = I was convinced / I felt certain

So this sentence does not express a casual opinion. It expresses strong confidence.

Because it is the polite past form of 確信する.

• dictionary form: 確信する
• polite non-past: 確信します
• polite past: 確信しました

The speaker is speaking politely and referring to a past moment of conviction.

A plain-style version would be:

• 彼が来ると確信した

It is understandable and correct, but it sounds somewhat formal or serious. In everyday conversation, people may more often say things like:

• 彼が来ると思った = I thought he would come
• 彼が来るって信じてた = I believed he would come
• 彼は来るはずだと思った = I thought he was sure to come

確信する is often used when the speaker wants to sound firm, deliberate, or formal.

Not naturally with 確信する in this structure.

The normal pattern is:

• ～と確信する

For example:

• 成功すると確信している = I am convinced that it will succeed

Using ことを would sound unnatural here. Some verbs take こと, but 確信する usually takes a clause + と.

The subject of 確信しました is omitted, which is very normal in Japanese.

So the full meaning is something like:

• I was convinced that he would come
• or we were convinced that he would come

The exact subject depends on context. In many cases, English needs a subject, but Japanese often leaves it unstated.

It is:

[embedded clause] + と + main verb

So here:

• 彼が来る = he will come
• と = that
• 確信しました = was convinced

Put together:

• 彼が来ると確信しました
• literally: [He will come] that, was convinced
• natural English: I was convinced that he would come

No, not in natural interpretation.

Because 確信する commonly takes a content clause with と, the sentence is understood as:

• I was convinced that he would come

If you wanted to say I became convinced when he came, Japanese would need a different structure, such as:

• 彼が来たとき、確信しました
• 彼が来て、確信しました depending on the intended meaning

So in your original sentence, と is not being read as when.