1. About the names of the provinces: in 1177 three sons of Wladyslaw II, duke of Slask (Silesia) and Poland, received sovereign duchies - Glogowskie, Wroclawskie and Raciborskie. So I suggest changing the names of three provinces: Lower Silesia (Dolny Slask) to Glogowskie, Upper Silesia (Gorny Slask) to Wroclawskie and Cieszynskie to Raciborskie. Admittedly Wroclaw is situated in Dolny Slask (so as Glogow) and Gorny Slask is absolutely other region of Poland, but Wroclaw lies more on east than Glogow, but more on northwest than Cieszyn, which after 1177 was merged into duchy of Raciborskie, part of Slask (Silesia). I also suggest changing the german name Danzig to polish Gdansk. But it isn't most important at the moment.
2. Poland absolutely shouldn't be a kingdom, Poland should be a vassal of Germany, although it will causes many problems with all duchies, which should be changed into counties with one liege...
3. Partition of country:
a)
Kingdom of Poland =>
Kazimierz II the Just Piast: Chelminskie (369), Krakowskie (capital, 527), Sandomierskie (531) Sacz (532), Czersk (530); titles: king of Poland, duke of Mazowsze. Vassals:
-
county of Kujawy (Kujawskie) => Bolesław: Kujawy (428)
-
county of Gdansk (Danzig) => Sambor Subisławowic: Gdansk (368)
-
archbishopric (solution likewise in 1066B) of Krakowskie => Gedko Gryfita (date of birth unknown, but ca. 1130 would be alright, I believe): Sieradzko-Leczyckie (528)
-
bishopric of Plock => Wit z Chotela (date of birth unknown, but dead in 1206 - I don't believe he lived longer than 60 years): Płock (529)
b)
Duchy of Slask (Silesia) => Boleslaw the Tall Piast: Upper Silesia (Wroclawskie, 435)
- county of Glogowskie => Konrad Piast: Lower Silesia (Glogowskie, 434)
- county of Raciborskie (Cieszynskie) => Mieszko the Tandleleg: Cieszynskie (Raciborskie, 526)
c)
Duchy of Wielkopolska => Mieszko III the Old Piast: Gnieznienskie (429), Kaliskie (capital, 432)
- county of Poznanskie => Odon: Lubusz (430), Poznań (capital, 431).
4. People
a)
Kazimierz II the Just (king of Poland) => daughter
Maria dead 1178,
b)
Boleslaw the Tall (duke of Slask) =>
Boleslaw born 1129, don't inbred; wife Vierkuslava =>
Viacheslava; son
Jaroslaw was a bishop of Wroclaw, so give him ecclesiastical education; son
Bolesław born 1181; son
Jan born 1163; son
Henryk I the Bearded born 1167, give him high stats (especially in stewardship and diplomacy - he was one of the greatest dukes of Poland all the time); daughter Adelheid =>
Adelajda, she was a wife of Depold III margrave of Morava, but this guy don't exist in the game; daughter Bertha =>
Berta; daughter
Olga => delete, never existed
c)
Mieszko III the Old (duke of Wielkopolska) => son
Odon born 1141; daughter Wierzchoslawa Ludmilla =>
Wierzchoslawa Ludmila; daughter Judith =>
Judyta, wife of Bernard count of Anhalt;
Mieszko the Young born 1160, dead 1193; son
Wladyslaw III the Spindleleg born between 1161 and 1167, so choose the year you want

; daughter
Anastazja born 1164, wife of Boguslaw I duke of Pommerania; daughter
Zwinislawa => delete, never existed
d)
Sambor Subislawowic (count of Gdansk) =>
Sambor born 1160; two sons (Sobieslaw II and Swietopelk), but the dates of birth unknown
e)
Konrad (count of Glogowskie) =>
Konrad born 1139
f)
Mieszko the Tandleleg (count of Raciborskie) =>
Mieszko born 1138; son
Kazimierz born between 1176 and 1179
g)
Odon (count of Poznanskie) =>
Odon born 1141.
5. Bogislaw => Boguslaw I (count of Slupskie) should be a duke of Szczecin (Stettin) and a vassal of Denmark.
6. Change the law system to ecclesiastical domination (from the convention in Leczyca, 1180, approved by the Pope 28 march 1181).
7. Mieszko III the Old (duke of Wielkopolska) should have a claim on title King of Poland.
8. Archbishopric of Gnieznienskie - there was probably no archbishop between 1180 and 1190 and I coundn't find enywhere, why.
Bibliography

"Poczet królów i książąt polskich", Czytelnik, Warszawa 1980
"Królowie i książęta polscy", Atena, Poznań 1997
"Encyklopedia Powszechna PWN" t. I, II, III, IV, V, VI, Warszawa 1995-1996