Alternative in parenthesis before or after?

Two rules apply here

1. The sentence should read correctly without the parentheses
2. The sentence should also read correctly if the operative part of the parentheses (i.e. excluding 'usually') replaced the comparison before it in the main sentence.

Rule 1 excludes sentence number 3, and sentences 4 and 5 are only allowed in US English where "different than" is a valid comparison. Rule 2 excludes sentence number 2 ("The ion temperature may be lower than from the electron temperature.") and makes sentence 4 only valid in US English.

So

• In UK English only sentence 1 is permitted (though sentence 5 could be made correct by substituting 'from' for 'than')
• In US English sentences 4 and 5 are correct, and possibly 1 if you permit 'different from'.