Sealed and guaranteed sure sound like done deals to me. If scripture wanted to make it clearer that salvation is a done deal, I'm not sure what wording they would need to use to convince you. I see no reason to limit the concept of a guarantee, since you describe a down payment as a pledge to pay in full. What is being paid in full at a later time? Not our salvation itself, but our inheritance. What is our inheritance? Rewards for deeds done, immortality, and being in the direct presence of God.Actually,I'm not how you see it as a done deal when the passage you quoted says one is sealed until the day of redemption. How is it complete if one isn't redeemed yet?
Christians also misunderstand the use of the word "guarantee." In this usage it doesn't mean that something will definitely happen as many suppose. A guarantee is a down payment, a pledge to pay something in full at a later time. If we look at the passage we see that the guarantee is a pledge to redeem the believe on the day of redemption.
I would also submit that there is no dichotomy between the Israel and the church as you suggest. That Jesus words are applicable to the church can be see in His letters to the churches in Revelation, 'to him who overcomes.'
Here's another way to look at it: If indeed we have the Holy Spirit in us, we are being transformed into the likeness of Jesus. Our righteousness is not our own but only found in Jesus. So the only way we could fail to receive our inheritance is if Jesus could do so, and the only way we could not be guaranteed to be already saved is if Jesus is not. We "were bought at a price" and are "not our own".
As for the church separate from Israel, that's a whole other topic. I have some writings that include this, if you're interested:
http://books.fether.net/index.php?theBook=BPFF&theChap=3