My take on this is that the scripture clearly states that salvation is synergistic, but this fact does not detract from God's sovereignty in the least... in fact, it shows he is bigger and more powerful.
Deuteronomy 30:1
So it shall be when all of these things have come upon you, the blessing and the curse which I have set before you, and you call them to mind in all nations where the LORD your God has banished you,
Deuteronomy 30:6
Moreover the LORD your God will circumcise your heart and the heart of your descendants, to love the LORD your God with all your heart and with all your soul, so that you may live.
Deuteronomy 30:11
For this commandment which I command you today is not too difficult for you, nor is it out of reach.
12 It is not in heaven, that you should say, 'Who will go up to heaven for us to get it for us and make us hear it, that we may observe it?
Deuteronomy 30:15
See, I have set before you today life and prosperity, and death and adversity;
Deuteronomy 30:19
I call heaven and earth to witness against you today, that I have set before you life and death, the blessing and the curse. So choose life in order that you may live, you and your descendants,
Clearly, these verses are in the context of Salvation (Circumcision of the heart). It says it is a CHOICE God sets before man. It explicitly states Salvation is NOT too difficult for man to make the choice, and that it is NOT a choice made by God in heaven. Paul quotes Deuteronomy 30 when speaking of Salvation in Romans 10:
Rom 10:6-10 NASB
6 But the righteousness based on faith speaks as follows: "DO NOT SAY IN YOUR HEART, 'WHO WILL ASCEND INTO HEAVEN?' (that is, to bring Christ down), 7 or 'WHO WILL DESCEND INTO THE ABYSS?' (that is, to bring Christ up from the dead)." 8 But what does it say? "THE WORD IS NEAR YOU, IN YOUR MOUTH AND IN YOUR HEART"--that is, the word of faith which we are preaching, 9 that if you confess with your mouth Jesus [as] Lord, and believe in your heart that God raised Him from the dead, you will be saved; 10 for with the heart a person believes, resulting in righteousness, and with the mouth he confesses, resulting in salvation.
So we know that Deuteronomy 30 speaks of Salvation.... so when it says (3 times) that it is a choice set before man, and when it says it is not a choice God makes in heaven, we know it speaks of Salvation. Moreover, Deuteronomy 30:19 shows the choice is completely synergistic - God calls "heaven" and "earth" as co-witnesses of the choice man makes.
Romans 8:16
The Spirit Himself testifies with our spirit that we are children of God,
17 and if children, heirs also, heirs of God and fellow heirs with Christ, if indeed we suffer with Him so that we may also be glorified with Him.
To "testify" means to "bear witness to" so Romans 8:16 shows that God's spirit (heaven) and our spirit (earth) testify (witness) to the choice man makes for salvation. Romans 8:16 and Deuteronomy 30:19 say the same thing.
Now as to Ephesians 1, it gives the order:
Eph 1:13-14 NASB
13 In Him, you also, after listening to the message of truth, the gospel of your salvation--having also believed, you were sealed in Him with the Holy Spirit of promise, 14 who is given as a pledge of our inheritance, with a view to the redemption of [God's own] possession, to the praise of His glory.
So the indwelling of the spirit comes AFTER listening to the gospel and believing it... not BEFORE.