How about I work through piece by piece and see if I can find an answer to that question.
Edwin son of John (chair maker) and Elizabeth Glasson of George Street baptised 10th June 1855 Kenwyn
From the 1861 Census I would think that John Henry was the same John Henry baptised 26th March 1826 at Kenwyn son of Robert Glasson and Mary Prout Reynolds.
Something you have not mentioned but which perhaps confirms the name of Edwin's wife:-
1881 Census
Denises(?) Cottage, Chittlehampton, South Molton, Devon
John ISAAC, head, mar, 53, mason's labourer, Chittlehampton, Devon
Emma do., wife, mar, 52, formerly washerwoman, Chittlehampton, Devon
Eliza GLASSON, daur, mar, 21, labourer's wife, Chittlehampton, Devon
Emma ISAAC, daur, 11, Chittlehampton, Devon
Samuel do., son, 10, Chittlehampton, Devon
The 1891 Census at Canton, Glamorgan, Wales shows Edwin's wife as Eliza, age 34, born Chittlehampton, Devon so there is further confirmation of her identity.
As you say, the youngest daughter is clearly Florence. The elder daughter's name is not clearly spelt but the way 'Eliza' was written I would suggest we are looking at 'Kerzia' which would in fact be 'Kezia'.
AND - they do not appear in FreeBMD.
I have looked up the birth entry for Eliza Isaac just to confirm the Registration District for Chittlehampton which is South Molton.
BTW Eliza was registered S. Molton December Qtr 1858 Vol 5b Page 431
The daughters cannot be found in South Molton R.D. by searching without the surname either.
As a last resort I have tried a search of the images from September Qtr 1882 through September Qtr 1883 looking for Kezia and there is absolutely no sign.
Most unusual for that time to have one birth not registered ..... but for two births in the same family to be not registered is something else again!
Perhaps it could only happen in Devon!
And it looks like Kezia might have been lost in the Bogs or on the Moors because after 1891 there appears to be no further trace of her!
CT