1) The "naa" in the words you have mentioned consists of a long vowel (which explains why it is double a) - أنا، هُنا and the suffix ـنا - notice the alif at the end, which stands for a long vowel. 2) No, the accent/stress is not influenced by the ة. 3) Yes, it is originally written هاذه but semantics opt for omitting the alif for an "aesthetically better" spelling + هذا has two long vowels as you said. 4) You can say أهلا وسهلا without nunation (tanween ً) in dialects, but the nunation is retained in MSA. As for شكرًا, the nunation remains regardless. 5) It is up to them if they shorten it, which is usually done colloquially, but does not change the fact that it is a long vowel. I hope this helps!