wanna ask something..hope my spelling is right..if not correct me pls

What is the following?
so shite
sore wa
shitemasu
dou ishitte
okage sama de

What is the difference between these two wakarinai and wakarimasen ? Also wakarimasen is i dun understand, but how to say u don't understand??

Do you say irashimase (welcome) after someone says, "arigato gozaimasu"?

What is okage sama de?

desu ne and desu ka difference ( e.g ii otenki desu ka? or ii otenki desu ne?)

sukoshi nihon go ga wakarimasu (does this means i understand a little japanese)

just wondering what is shitsuji desu/ shitsuji kara

i always hear when ppl say watashi wa xxx, shitsuhi desu ,etc

u mean its someone's name??? arigato