You can use one ETC per person.

One-way flights on most international routes are much more than half of a roundtrip. You probably won't to book them. (You definitely won't want to book a one-way business class fare).

Current fares for those dates:
Coach (lowest), roundtrip: $1657 pp
Coach (GPU-eligible), roundtrip: $1809 pp
Business (lowest, 6+ hour layover at IAH on the return), roundtrip: $4842 pp

Coach (lowest / GPU-eligible), one way: $971 outbound + $1279 return = $2250 pp
Business (lowest, 6+ hour layover at IAH on the return, one way: $3652 outbound + $3817 return = $7469 pp