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Please have a look on these expressions:
The fragile peace he helped maintain between the various Italian states collapsed with his death.
1grace: 1a: unmerited divine assistance given humans for their regeneration or sanctification

I would have put a 'to' in the middle of those bold units because I find it more natural. Would it alter the meaning if 'to' had been used? Could you please guide me of a general rule about where 'to' is/could be omitted? Thank you.
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Jackson6612Could you please guide me of a general rule about where 'to' is/could be omitted?
There is no rule that is always applicable. Re your examples, two "rules" apply.
Jackson6612The fragile peace he helped maintain between the various Italian states collapsed with his death.
To is optional after help used in the active voice:
I helped him [to] do it.
If help is in the passive voice, to must be used:
He was helped to do it.
Jackson6612unmerited divine assistance given humans for their regeneration or sanctification
To is optional after given if given is part of a passive structure:
Money was given [to] him.
In your second example given is part of a passive structure. That is obvious even though you haven't provided a complete sentence. A more complete version could be: ... unmerited divine assistance [that was] given [to] humans for their regeneration or sanctification...

CB
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Thanks a lot, CB.