Learn English – Correct use of ‘not be able to’

You got both right. "Ever" modifies "have seen" so it goes before that and follows "not". However, I often see it between "have" and the verb, as in: "Jan can't have ever seen a mobile phone before."

The second is fine. In that sentence, the adverb "undoubtedly" could actually be anywhere and not really change the meaning. It modifies the entire phrase "must have been treated badly" and not just one particular word in that phrase. Usually, adverb placement will change the tone and emphasis based on its placement but that depends on context.

At the first chance is occasionally found in contexts like this, but as J.R. suggests it is rare, and it feels alien to my ear. The Google Ngram below appears to corroborate this; as you see, chance occurs in print only about 5% as often as opportunity—and some of those occurrences are "false hits", things like He leapt at the first chance.

Note, however, that chance is considerably more frequent when it takes a complement such as at the first chance to meet you or a modifier such as at the first chance he got.

Nonetheless, I would avoid chance in these contexts and write opportunity instead:

I will come and meet you at the first opportunity.