Japanese - (Words \ verb endings)
Hello

I am introuble about this japanese words "iikagen, doumo, gomennasai, sumimasen".

With "iikagen" I only know an idiomatic phrase: Iikagen ni shiro (That's enough), but I think this is used without this idiomatic use, but I don't know hou to use it.

I know that "doumo" means "thanks", but I heard this phrase: Maitta ne doumo.
I thinkt this "doumo" is not "thanks". Can thiis be used as a final particle?

I know that "sumimasen" can mean "I´m sorry", but "gomennasai" means "´I'm sorry, too". Which one can I use? Is there any difference?

Using "Past + ato de" and "Imperative + kara" means the same thing in english. Is there any difference between them?

Asa gohan wo tabete kara, gakkou ni ikimasu.
Asa gohan wo tabeta ato de, gakkou ni ikimasu.
(Both mean "After eating my breakfeast, I'll go to school.)

The same thing happens with "Imperative + shimau and Verb + owaru".

Kono hon wo yomiowatta.
Kono hon wo yonde shimatta.
(Both mean "I have finished reading this book.)

Question

Japanese - (Words \ verb endings)
Hello

I am introuble about this japanese words "iikagen, doumo, gomennasai, sumimasen". 

With "iikagen" I only know an idiomatic phrase: Iikagen ni shiro (That's enough), but I think this is used without this idiomatic use, but I don't know hou to use it. 


I know that "doumo" means "thanks", but I heard this phrase: Maitta ne doumo. 
I thinkt this "doumo" is not "thanks". Can thiis be used as a final particle?

I know that "sumimasen" can mean "I´m sorry", but "gomennasai" means "´I'm sorry, too". Which one can I use? Is there any difference?

Using "Past + ato de" and "Imperative + kara" means the same thing in english. Is there any difference between them?

Asa gohan wo tabete kara, gakkou ni ikimasu.
Asa gohan wo tabeta ato de, gakkou ni ikimasu.
(Both mean "After eating my breakfeast, I'll go to school.)

The same thing happens with "Imperative + shimau and Verb + owaru".

Kono hon wo yomiowatta.
Kono hon wo yonde shimatta.
(Both mean "I have finished reading this book.)

Answer

1. iikagen

iikagen as a bad meaning refers to inaccurate or irresponsible or something like that. So, you can say "Kare wa iikagen na hito da" etc.
Also, iikagen is an adverb like "pretty" or "rather". For example, "iikagen unzari da." It means "I'm quite/really fed up."

2. doumo
Doumo is a kind of interjection. Maitta ne doumo you mentioned  is one of the examples.

3. sumimasen
Sumimasen is more common, and gomenasai is more casual, but the difference is small. Gomenasai is used among women more often than among men. It sounds more intimate.

4. atode and kara
Your example sentences are exactly same meaning.

5. shimau and owaru
Yomiowatta implies that you just  mention the fact that you fininished to read.
Yonde shimatta sounds like you have some feelings. It implies you finished to read and you felt something else, such as you're a little bit sad because you can't read it any more, etc.
But those two are basically same meanings.