Learn English – To make a long story short, formal or informal

"However" functions as a more formal version of adverbial "though", this mainly due to an informal appeal to the reader that "though" suggests when it comes at the end of a sentence. On a different note, it doesn't always imply a direct contradiction. It could simply imply a qualification or contrast to what has already been stated.

For example, you could use it to qualify something to avoid confusion.

"I'm sick. I still would like to meet for coffee, though".

On the contrary, you could use it to contradict a previous statement.

"My boss told me not to come to work because I was sick. I had too much work to do to stay home, though".

In your example,

"How do we know it is a bicycle, though?"

might not (we'd need more context) contradict whether it is a bicycle or imply any contrast, but rather seek to qualify its bicycle-ness.

If you were compelled to change the wording, here are three options that incrementally emphasize the qualification:

"It is a bicycle, but how do we know it is a bicycle?"

"It is a bicycle, yet how do we know it is a bicycle?"

"It is a bicycle; however, how do we know that it is a bicycle?"

I would avoid, where possible, words like 'rather' and 'quite', in the sense of 'quite good'. They are imprecise and overworked. In any case, if you say you got 'very good' comments, using 'rather' and 'quite' may mean you are tending to be too modest.

So for starters you could say:

1. We received very positive reviews.

The word 'appraise' is used more in the sense of 'evaluate'. In your case I would use its root 'praise'.

1. Our results were praised by the reviewers.

   'Affirmative' hints at something being positive but doesn't really say it directly. I think you need to say they were affirmative of something. How about:

2. The reviewers comments affirmed the strength of our results.