Have sealed (sphragisamenos). First aorist middle participle (antecedent action, having sealed) of sphragizō, old verb from sphragis, a seal (Rom 4:11), to stamp with a seal for security (Mat 27:66) or for confirmation (2Co 1:22) and here in a metaphorical sense. Paul was keenly sensitive that this collection should be actually conveyed to Jerusalem free from all suspicion (2Co 8:18-23).
I will go on by you (apeleusomai di' humōn). Future middle of aperchomai, to go off or on. Note three prepositions here (ap' from Rome, di' by means of you or through you, eis unto Spain). He repeats the point of Rom 15:24, his temporary stay in Rome with Spain as the objective. How little we know what is ahead of us and how grateful we should be for our ignorance on this point.