SharePoint : How to make a field mandatory based on another field, in a datasheet list view?

You can use List Settings -> Validation Settings for this.

e.g. =IF([Replacement Available]="Yes",IF([Replacement]="",FALSE,TRUE,TRUE)

This formula would affect the default 'new item' form as well.

SharePoint InfoPath Form: How to ensure that a text field contains at least 100 characters?

Specify a validation rule for the field as below:

Here 'JD' is the field name.

Condition is set to "does not match the pattern"

Pattern: .{100,}

A . would allow the user to type in any character as long as the length is at least 100.

SharePoint List Validation: How to validate a column based on the value of another field in the list?

=IF([Column1]="SomeValue",IF(ISBLANK([Column2]),FALSE,TRUE),TRUE)