"Hello and thank you, friend."
Translation:Halò agus tapadh leat a charaid.
No, that’s completely unrelated.
What you ask about in this sentence – the difference between leat and leibh – is the difference between singular informal form (leat) used when speaking to your peer with whom you’re on first name basis, and plural/formal form (leibh), used when speaking to more than one person, and when speaking to a stranger, an elderly, etc. (used to show respect or keep distance).
What you linked is about the attributive adjectives (adjectives describing a noun they stand next to) and their lenition (changed pronunciation marked by added h after the first consonant) – there are no attributive adjectives in the sentence of this thread.
caraid is the basic form (nominative-accusative), eg. tha an caraid anns an taigh ‘the friend is in the house’ or chí mi caraid ‘I see a friend’
But when you address people in Gaelic, you need a vocative, and words starting with a consonant form their vocative using a particle a before them and lenition of their first consonant – so when speaking to your friend you’ll address them by a charaid.
There is no particle a before vowels (so vocative of Anna is just Anna).
Some words (especially masculine nouns) will have some other changes in vocative too (eg. Seumas ‘James’, a Sheumais ‘(oh, my) James!’ – this vocative has been borrowed back as Hamish into English).
And lenition will happen in other situations too (eg. in nom. of fem. nouns after the definite article: bò ‘a cow’, but a’ bhò ‘the cow’).
WINDOWS: If you are on Windows you can modify the keyboard settings (press the Windows key, then type "keyboard", then select "Edit Language and Keyboard options" from the list). The extended UK keyboard or Scottish Gaelic Keyboard will work. Once installed, you should get an option next to the clock that allows you to change language/keyboard setting at will. Once you select the UK Extended Keyboard or Scottish Gaelic keyboard, then when you press the back-tick ` prior to a letter with an accent it gains the accent when you press the subsequent letter. e.g `+a = à, `+e = è, etc.
ANDROID/IPHONE: If you are on iPhone or Android you can long press a letter on the keyboard (standard keyboards, if you install a third party keyboard it may do something else) then extra versions will appear above the key that you can select. The vowel with grave accent is usually the first in the list for my iPhone.
Both. It’s just regional variance of the strength of preaspiration.
c, t, and p at the end of a syllable in Scottish Gaelic often are preaspirated, ie. pronounced as if written chg, chd, chb or hg, hd, hb – with a ch /x/ or h /h/ sound before them – but the strength of the preaspiration differs in different regions. Most speakers will strongly preaspirate c (so mac son will commonly sound as if machg), but t will often have weaker preaspiration, and many speaker will have only slight preaspiration of p or no at all.
See this article on the Akerbeltz wiki: http://www.akerbeltz.org/index.php?title=Pre-aspiration_or_What_the_h_in_mac_is_about
and this on Wikipedia: https://en.wikipedia.org/wiki/Scottish_Gaelic_phonology_and_orthography#Preaspiration
Both give a rough map of the regional differences.