I realised that might be a problem. However, the acute, grave, circumflex, and breve are all tone marks. If those were all gone, it would look rather like Tas.

What does the diaeresis mean?

The diaresis makes o /o/ and u /u/ into ö /ø/ and ü /y/.

Where do the front rounded vowels come from when they appear to be infrequent in the language, and not motivated by a process of vowel harmony or something similar?

In Erhain:

1. E aradde carnáiad ru-ndärat win onal.
2. Amariod fir wi calniod nalad-moraniondan. Dan ansiláiod innan tornde Sinear sacel amer, wo niod ban masal. calnáiod därat mïlan sinandan wi simiod hurn mun-firil.
3-4. Canáiod nälar sacail camøs tır amer. E tirde niod camál picin amar arliais, ni riniod safarde. Consin atan ämarcar. "Saiar hováian hyrn anhirion däraténacar, fon na-ntoráion dalmiar atan parril", oronáiod. Wi saiar cemar tiir anviod. Simáiod brannil münsin ämar wi hannáiod co siman safel.
5-6. Win amiod Tarnan cem tiir wi piciin, saniod: "amano ni barnáiod, ennic esde anonnar nutairéda. A fircar win-onalnun, niade senir amin namaran nur!
7. Hinfario wi gadio dïnat önel ïtärıc, fon na-rsallanáiad-nur enser!"
8. Saiar disiviod Arn däret alamwen wi singáiod camosnun tiir.
9. Anim onomiod e tir Babel (Wissin). niod e hurn, wo disiviod Tarnan wissın däratdur, gadic dïnat önel ïtärıc wi disivic döet run'alamwen.

Monopthongisation of /eu/, /oi/, /ui/, /oe/, and a bunch more. I can't remember them all. Example, the initial vowel marking causative distant past is öó (supposed to be ö but high tone) from earlier oe. The high tone came by analogy with /é/ and /ó/ as the markers for distant past and causative, respectively. The high tones in the last two came from /eː/ and /oː/, so guess what type of language that means this is...

Why not write these vowels as ǿ and ý? That seems more esthetic.
Um, I don't know. Tonal?

I could do that, but isn't ǿ not supported by many computers? Ý works though, and I should start using it.

And e: and o: are ablaut patterns common to Indo-European languages, which Jinnic (actually, the Anglicised name is Mesis) is.

1. Nunha la tera tenhida un lingua e los lemos mesmos.
2. Na duração da migração de ominos do ésto, trovalhadan un campa na tera de Shinar e álas se instalada.
3. Pois que diguedan a un e a la oitra que, "Venhe, que falhen las masõas, pois que les brulhar aprofondamente." Tenhen las masõas peros pedros, e la betuma pero mortero.
4. Pois que diguedan que, "Venhe, que construcir a nós un vilha e un tora con lo suo verto no celho, e que dar a nós un titro, no visagem da cança da nostra dispersão as todas regiões á tera."
5. E lo Dio venhida asu a ver la vilha e la tora, de qui folhen bastidas peros infantos do raço de omino.
6. E lo Dio digueda que, "Aqui ver, tenhe solà unas pesõas, e elhos todos tenhe la mesma lingua, e se é solà la começagem de qué elhos valhan far. E nada que elhos prometen da nunha estará imposivera a elhos.
7. Venhe, que asu ir e far la confusão das suas linguas porque nã poden comprender la ditação a un e a la oitra."
8. Pois que lo Dio les dispersada a toda la tera, e lasadan bastir la vilha.
9. Perse lo Dio se quilhada com Babel, porque álas lo Dio falheda la confusão entre todas á tera. E dálas lo Dio les dispersada a toda la tera.

Nidel Pivio--Zhenkoi

1. Lede dimal nulli e nyendeli gilusi at ciru deli.
2. At ic pether relesi iston, kotese nupe ot de nule Shinaro, at dara cunasi.
3. Ciasi ushedater, "Seluno, zh'uneru rikura ploki at tal nezhuuk pisuru." Plok otnui lioco acusi, at nim kel renu.
4. Dute ciasi, "Seluno, zh'uneru pamora nyencur senoi doth nidel dak geveter lishuse, ce dak riora rekura tilu kel nyencur, at tanisi cenili eju kisun dimalo nullio."
5. Pema amun ex selule, senoi at nidel cuan dak rimer pamosin.
6. Amun ciase, "Ok ic e pether cabase ciru nyju kisun dimalo nullioendeli pusin ve vean, dute tanengo bemise vean talte orecule iso."
7. "Seluno, zh'uneru ex hese at setakuse kat nyendeli, ce tan alakinoso ushedar.
8. Ce amun cenilisi ket daran eju imm nulli, at cesi senoi pamosin.
9. Dak wai ise simosin Pivi--cotec dara amun setakusi nyendeli dimalo nullio. Daran amun cenilisi kat eju kisun dimalo nullio.


This is my language i have been developing for about seven years, please, don't drool on your keyboard, this has been a long time coming...

so let me know what you think please any comments or criticisms will be greatly appreciated.