I believe the correct SWF spellings are a wre’ta, ny wre’ta, fatel wre’ta --- a vyn’ta, prag y fyn’ta, ny vyn’ta; my a vynsa --- ny yll’ta with two LL in forms of gallav but one N in the forms of mynnes that you mentioned. I’m basing that on the sample verb conjugations in the document Specification Final Version.pdf aka An Outline of the Standard Written Form of Cornish.
You will often see vynn’ta, fynn’ta, mynnsa used which is, I believe, the correct spelling in Kernewek Kemmyn which many Cornish speakers/writers are more used to than SWF but I think those are not SWF spellings. (The nn versus n spellings in SWF usually depend on the Late pronunciation which is men’ta, menja here rather than mednta, mednja that would be expected for an SWF spelling with nn. Some other forms of mynnes do have dn in Late Cornish and thus nn in SWF, e.g. the infinitive mynnes itself.)
Note also prag y fyn’ta with y (pronounced like a shwa here, I think, since it’s an unstressed particle) rather than prag a fyn’ta with a.
Also, ny wrav vy has no apostrophe; the -v is part of the verb form itself, not a pronoun.
And I think that the past tense of dalleth is my a/re dhallathas with -a-, though I have no way to check the SWF spelling of this; there is, sadly, no SWF book of verb conjugations yet so except for the dozen or so verbs conjugated in the SWF specification itself, one can only guess. If the Unified Cornish verbs book and the Kernewek Kemmyn verbs book agree on a spelling, then presumably that will be the SWF spelling as well, but if not, then…
On the other hand, since SWF only wants to standardise spelling, not grammar, presumably in case of disagreement, both spellings (possibly with slight modifications to fit SWF principles) should be valid Cornish.
As for ny wre’ta versus na wre’ta: na wredh (without ’ta, and so with full verb ending) would be a translation of ‘no’ as an answer to a question but if you then go on to make a statement, it would start with ny wre’ta. (na is also used in negative relative clauses such as ‘The book that I do not write’, an lyver na wrav (vy) y skrifa, but I presume SSiC doesn’t teach that sort of thing just yet.)
Feel free to ask me other things; Cornish hasn’t been at the forefront of my language efforts but I did study it for about two years, passed the Cornish Language Board’s third exam, and like to think I have a reasonable handle on it, especially on SWF spelling.