When a magus delivers a touch spell as a free action using spellstrike while employing a weapon, is his extra damage from his Strength bonus halved and his bonus damage from the feat Power Attack as per an off-hand weapon?
No, you're delivering the spell through your weapon that is in your main hand. That language you're reading is to clarify that "you're using your offhand to cast a spell so you can't be holding anything else in it". So you get none of the penalties you're specifically asking about.
If there's anything about the magus still confusing you I highly recommend that you read both walter's guide to the magus and this guide on the paizo rules forums (the latter specifically covers the intricacies of magus spell casting/fighting)