Sandhi (Joining Sounds)

"Vidya" and "aalaya" are to be joined. Ravi writes "vidyaaaalaya", keeping both words exactly as they are, side by side. His sister says the two vowels combine into a new form here. Who is right?

Joining "dev" and "indra", Meera thinks the same rule as the earlier example ("aa"+"aa"="aa") applies, and writes "devaandra". Her brother says a different rule applies here. Who is right?

Joining "Ram" and "avataar", Tanu writes "ram-avataar" (kept separate), thinking nothing changes between two consonant-ending-feel words with "a" between them. Her teacher says the vowels combine here too. Who is right?

"Su" and "aagat" are to be joined into one word. Disha thinks a rule like before, turning a vowel into "aa", will apply, and guesses "suaagat". Her friend says a vowel here actually turns into a consonant. Who is right?