‎اِسْتِثْنائِيّ is totally correct .no need to ء under ا . the rules are a long story .but this link maybe helps you (it's written in Arabic ) https://mawdoo3.com/%D9%85%D8%A7_%D9%87%D9%8A_%D9%87%D9%85%D8%B2%D8%A9_%D8%A7%D9%84%D9%88%D8%B5%D9%84_%D9%88%D9%87%D9%85%D8%B2%D8%A9_%D8%A7%D9%84%D9%82%D8%B7%D8%B9 and yes you have to write ء if it was needed even though the words still understandable to native Arabic speaker without ء. But it will be a written mistake

At the beginning of certain words, the hamza shouldn't be written on the alif. This is called /hamzat alwaSl/ (همزة الوصل), e.g. in words like two (اثنان), Monday (الاثنين), woman (امرأة), etc. It's very confusing, and it's applicable to me sometimes.

I think that I've already answered you to this question, and that’s not the end of the story, there's more to follow on hamza.

Hi Magdalena, thank you for your help!

Hi. One woman here helped me a lot, when I had similar question. Have a look at it: ' Stems VII to X (let's disregard the rare stems from XI upwards) start with اِ, i.e. alif with a kasra. In these stems, you never write a ء wherever you have a form that starts with "i", i.e. all past tenses, all imperatives and the masdar. It is a very simple rule. Please note that many Arabs err in them, and some will even correct you wrongly'.