Senare is the comparative form of sen, which came from the Old Norse seinn. Sayônara was pronounced as sayou naraba during the 18th century, and could be more different in earlier centuries.
Lastly, I doubt that there was any form of contact between the Vikings and Japan during the Middle Ages XD
The difference is very small. If you say ska, you express a little more determination or intention: you have decided to call. But the difference is small.
It's probably more common to say just ringer here in Swedish, but both ways are fine. You can also use our other future construction and say Jag kommer att ringa dig, but we haven't taught the future tenses yet at this point.
Whats wrong with 'I ring you later', I know it doesn't sound very natural in English, but semantically why isn't it correct?
So with you saying that present tense is natural in Swedish, does that also mean something like 'Jag får ett arbete' is also natural, compared with 'Jag fick ett arbete'?
fick is past tense so I'm not sure why you'd want to compare those two. A sentence like Jag får ett arbete would most likely be interpreted just as present tense/something general since there's no reason to think it should be future. In the sentence above, there's senare which shows it's in the future. Also, får is not something you can really decide for yourself (more so in Swedish than in English: not every English get should be translated into Swedish får). Normally we only use verbs that include a component of intention as the present-for-future, and får does not include any intention.