It is interesting what you say about the idea the of Seiunchin and Seipai Katas once forming a single unit of practice (that is a 'single' Kata). This is exactly the same situation with the three Longfist Forms in our family Hakka style - which contain 32 movements each - but once was practiced as a single Form containing 96 movements! To assist the investigation we can have a look at the etymology of the names of these two Goju Ryu Kata and see if any evidence presents itself:
Kata - Seiunchin = 制引戦
制 (Chinese) = Zhi - 制 (Japanese) = Sei - Control, Plan, Regulate, Law, Limit
引 (Chinese) = Yin - 引 (Japanese) = In - Draw (a bow), Pull, Lead, Stretch, Admit, Evade (Leave)
戦 (Chinese) = Zhan - 戦 (Japanese) = Sen - Battle, Fight, War and Conflict
Seiunchin Kata = Careful Bow-Drawing in Battle
Chinese Name = Zhi Yin Zhan (制引戦)
Japanese Name = Sei In Sen (制引戦)
Okinawa Name = Sei Un Chin (制引戦)
Kata - Seipai = 十八手
十 (Chinese) = Shi - 十 (Japanese) = Ju - Ten, 10, X, Perfection and Utmost
八 (Chinese) = Ba - 八 (Japanese) = Hachi - 8, eight, VIII, Divide and Differentiate
手 (Chinese) = Shou - 手 (Japanese) = te - hand, open (hand), grip and handle
Seipai Kata = Eighteen Open Hand
Chinese Name: Shi Ba Shou (十八手)
Japanese Name: Ju Hachi Te (十八手)
Okinawa Name: Seipai (十八手)
It looks to me that the last word of the written name '十八手' (Shi Ba Shou) is missing in the way the Kata is named in the West. It seems that the Okinawan 'Seipai' equates to the Chinese 'Shi Ba' (Eighteen - as in '10' + '8') - but that the ideogram '手' (Shou) is missing from the name. The Okinawan name should probably read 'Seipaisou' or something similar. We are probably seeing the Fujian dialect preserved through the Okinawan language - with a descriptive word missing when the Kata is vocally discussed! I do not know why this is. What might be of significance is Pan Yu Ba's 'Arahant Fist' (羅漢拳 - Luo Han Quan) - which is also known as 'Arahant Eighteen Hands' (罗汉十八手 - Luo Han Shi Ba Shou).
Does the Seipai Kata represent the 'Arahant Fist'? Even so, there is the closed 'fist' (拳 - Quan) and then there is the 'Open Hand' (手 - Shou). As you know, Sensei, both types of hand are always used - together with the 'palm' (掌 - Zhang). When I was in various Buddhist temples in China, most had statues of the Eighteen Arahants (all enlightened visitors from India thousands of years ago) spread around the periphery of the grounds facing outward with each statue holding different positions with their hands and feet (this sometimes reminds me of Tensho Kata).
I spoke and practiced with Warrior-Monks and Nuns who all talked about rebirth, karma and 'meeting' one another again in different lifetimes. There was no violence at all with the flowing hand and foot movements 'evading' every violent movement. However, these people 'sensed' exactly when greed, hatred and delusion was present in the mind of the opponent - and they immediately took action to dissolve it with loving kindness, compassion and understanding! Most of these 'Arahant' Forms involve 'evasion' and simply 'not being there' rather than any forceful application. When power was needed, however, it was generated a) from the ground, and b) from the body-mass of the opponent - as their bodyweight was momentarily 'borrowed' before being 'given back'!
What we might be looking at is a combined Goju Ryu Kata entitled 'Arahant Eighteen Hands Regulate and Drawing the Battle Bow'! It could be named like this '羅漢十八手制引戦' (Luo Han Ba Shou Zhi Yin Zhan).