I can only deal with this piece meal but a major problem is we are dealing with those in the acting profession who are known to be highly mobile and not very constant when it comes to keeping the names they were given at birth.
Gertrude Grace Smythe born c1892 M Cert lists John Grantley Smythe as her father. That is believed to be false, but if the birth year is correct, John's death in Southam district in the December quarter of 1893 does make that a possibility. (this information conflicts with your earlier findings where you suggest John had died before a possible date of conception)
Until now, and for reasons shown later, John has rightly been thought of as a grandparent, as has his wife, Gertrude Chisswell, who he married in Dover in 1868. However, on the later censuses John's widow Gertrude indicated she was the mother of the Gertrude Grace Smythe born c 1892. Again not impossible, but it would have meant Gertrude Grace Smythe arrived when she already had another daughter from her previous marriage who was also called Gertrude (Gertrude Chisnall junior).
Note on age: Gertrude Smythe senior says she was 54 in 1901 and 34 in 1881 (giving a YOB c1846) making her 46 when Gertrude Grace Smythe was born and about 15 at the time of her first marriage. (Born 1846 married 1861)
The 1881 census has Charlotte Agnes Smythe from the above union who was born in Weymouth 1869, and two other children from Gertrude's previous marriage.
Gertrude Smythe's previous marriage was to Arthur Henry Chisnall (not Chisswell) in Brentford in June quarter of 1861 when she could only have been about 15 (if you go by the census). Her maiden name was given as Gertrude Johnnes Morse.
(could that be Johanna and was she older than she said on the census returns?)
Have you got her as Morse, or them as Chisnall/Chisswell on the 1861 census?
The 1881 census also shows that her earlier children included Gertrude Chisnall (born Kingston district 1862) & Edith Chisnall born Kensington 1864, but makes no mention of an Ernestine whose birth was also c1864. It transpires that Edith prefered to use her second name of Ernestine.
Gertrude Chisnall (born Kingston district 1862) the eldest daughter married in St Giles in 1882.
Edith Ernestine Chisnall married Michael Hart in Marylebone in 1883 and although Michael is said to have died prior to 1893, I have yet to verify that. (Too Many Micheal Hart deaths on FreeBMD)
The point has already been made that it is highly unlikely that Gertrude senior should have two daughters both called Gertrude, one born in 1862, the other in 1892, but there is also uncertainty whether Gertrude 1892 was an illegitimate daughter of Ernestine, concieved after the death of her husband Michael. There was a birth recorded on the GRO index for a Gertrude Grace Hart (Lambeth Sept quarter 1892) that would certainly fit, but you say you have a copy of that and the details are such that you felt it not to be the Gertrude you seek.
(Verify whether you have Gertrude Grace Hart bn Lambeth district 1892 or Gertrude Beatrice Hart bn Islington District 1891)
The Gertrude born c 1844-1846 mother or grand mother of Gertrude Grace Smythe, declared on the censuses that her birthplace was Brighton and we know from her first marriage that her birth name was Gertrude Johnnes Morse. But there were no Morses born in Sussex 1840-1848, and neither the IGI nor the GRO has any Gertrude Morses registered as born anywhere in England in that period. So not only is the grand daughter Gertrude born c1892 a problem, the grand mother who said she was born Brighton 1844-6 is as well.
Perhaps finding Edith Morse born c 1827 with her daughter Gertrude? born c 1844-6 on the 1851 census would be a useful start point to begin to unravel the the twists and turns in your difficult problem.
Lastly, you have said the 1892 Gertrude "Smythe" was also known as Grace. There is another researcher who may interest you on Genes Reunited, for in her family tree she has a Grace Smythe born in an unknown location c 1892,
Looking back, a can of worms comes to mind!!!!
Roy G