Soler-Espinosa of Spain had been targeting her first ever WTA singles title after reaching the semi-finals in Bastad, but Barthel, the highest-ranked player in the last four, triumphed 6-2 4-6 7-5.
Many of the higher seeds crashed out in the early rounds, with the likes of Camila Giorgi, Alize Cornet and Maria-Teresa Torro-Flor all out of the running by Thursday.
Kaia Kanepi had been installed as the favourite in certain quarters before the quarter-finals, but Barthel found a way past the Estonian and put in another determined display against Soler-Espinosa on Saturday.
Barthel appeared to be cruising to victory after the first set, in which she broke Soler-Espinosa twice.
The Spaniard also lost her first service game in the second set, but responded superbly to force a decider.
However, it was to be world number 61 Barthel who ultimately prevailed, thanks to a crucial break at 5-5 in the third set.
South African Scheepers needed just one hour and 15 minutes to dispatch Slovakia's Jana Cepelova, winning 7-5 6-2.
Proceedings were tight in the first set, but that was partly down to some unimpressive serving, with each player being broken on three occasions inside the first 10 games.
Scheepers ultimately found a fourth break and then held her own serve to clinch the first set.
And Cepelova faded thereafter, enabling Scheepers to reach her first WTA final in nearly three years.