I make some modification in my multi-origin headcanon for his activities and motivations.
A timeline for his activities and travels prior to Ostagar:
Late Justinian, 9:30: Events happen in Dwarf Noble Origin. Aeducan is arrested and a weeks long investiagtion and trial follow. Eventually he is found guilty.
2nd, Solace: Duncan recruits Daveth over strong objection from City Guard Captain of Denerim.
3rd, Solace: Events happen in Mage Origin. Amell is put under house arrest, confined in her own room. Fearing the Grand Cleric will order a much more severe punishment for her (i.e. execution or tranquility), Irving wrote a letter to Duncan, asking if he could recruit Amell into Grey Warden.
26th, Solace: Duncan at Orzammar, but finds no recruit. Aeducan has already been exiled into Deep Roads (later collected by Legion of Dead).
6th, August: Duncan at Redcliffe and recruits Jori.
14th, August: Cailan and his main army leave Denerim.
20th, August: Urien, Arl of Denerim, leave Denerim with his own soldiers, leaving his son, Vaughan, to govern the city in his stead.
21st, August: Duncan returns to Denerim and receives Irving's letter, thus he begins to prepare a journey to Circle Tower.
23rd, August: Tabris' wedding day, events happen in City Elf Origin. The Guard Captain, who already holds a grudge against Duncan, refuses to release Tabris, claims that only the king could pardon her. If Cailan doesn't choose to do so, then Arl Urien, Vaughan's father, will decide Tabris' fate. She is then thrown into the prison.
24th, August: Duncan leaves Denerim after a failed effort to secure Tabris' release. He travels to Fereldan Circle.
30th, August: Duncan arrives at Highever Castle. Teyrn Bryce has heard Grey Warden might have an eye on his kid. He believes it's Fergus. That's why he hurriedly sends Fergus away. He then offers Gilmore to Duncan as a replacement. The entire matter is, however, only a rumor. Previously Duncan has no plan to recruit someone from such a high-ranking noble family.
1st, Kingsway: Howe's attack on House Cousland. My Cousland is not in a condition to fight, thus having to rely on her mother and mabari for protection. Duncan helps to kill the last group of Howe soldiers, thus saving her life. Emboldened by this contribution, he demands Cousland to join Grey Warden as a show of gratitude, which leads to big quarrels between them. Still believing he could recruit a mage, he agrees to let her decide that matter once meeting with her brother.
6th, Kingsway: Duncan and Cousland arrive at Circle Tower. But the Knight Commander, who doesn't like Grey Warden's meddling with Circle affairs, says only King and Grand Cleric could pardon Amell. Duncan once again leave empty-handed.
12th, Kingsway: On their way to Ostgaar, Duncan rescued a delirious dalish elf, Mahariel. Events in Dalish Origin. Mahariel is persuaded by his friend Tamlen to explore a cave. The reason Tamlen insists to do so, even against Mahariel's objection, is because Flemeth gives him subtle suggestions in his dream. After staying briefly with Mahariel's clan, Duncan and Cousland leave with Mahariel.
17th, Kingsway:
Morning: After they arrived at army camp, Cailan extorts Cousland to join Grey Warden because of his own motivations. Cousland, who believes Duncan encourages him to do so, now actively hates both. Nevertheless, because Highever contingent is not powerful enough to fight Cailan and Loghain's men (and darkspawns) simultaneously, she has no option but to join, in order to give her men a maximal chance to survive.
Afternoon: Gathering darkspawn blood in Korcari Wilds. Flemeth shows up with Morrigan and gives a personal interview to both Cousland and Mahariel.
Dusk: Joining Ritual. Cousland and Mahariel survive. The other two die.
Night: Battle of Ostagar. Both Duncan and Cailan perish. Flemeth arrive at top of Ishal Tower to pull two wardens away from the impending death, because in her dragon form she could only grab two.
Note: My Tabris is set at a very young age, not yet 16. One reason Cyrion decides to marry her off at such a tender age is exactly to keep Grey Warden from recruiting her. Duncan is unable to stop the wedding, bebause the Hahren is one of very few friends he still has in Ferelden. Nevertheless, he thinks Tabris might not be happy with the wedding. If she decides to run away, he might still have a chance to collect her, that's why he shows up in Alienage. In the end, she doesn't try to run, but then Vaughan provides an unexpeced help.