'Additionally' doesn't have the same meaning as 'likewise', but rather it means 'also'.

'Similarly' and 'as well' have the same meaning but the latter requires a slight modification of syntax :

'In that situation, I would do likewise'

'In that situation I would do similarly'

'In that situation I would do that as well'

The most usual way of saying this, however, is '<em>In that situation I would do the same</em>' ; En esa situación yo haría lo mismo.

Likewise is rarely used by native English speakers. I would consider it informal but outdated and almost never used in conversation. Similarly, as well, and additionally have similar meanings but are all much more common.

The usage of likewise has declined 93% since 1800.

That means that it is not used very much anymore.

"likewise" is often used informally to mean "me too" and it is often used as a single word response,

e.g.,

A: I like chocolate ice cream.

B: Likewise.

Yes, it would be alright to use likewise that way. However, more common responses in conversation would be “neither do I,” “same,” “me either,” (poor grammar but common in spoken American English).

I don't really use it much at all.