Translation:I agree with you that we should try again.
According to Whitlam's Modern Brazilian Portuguese, the use of "dever" covers a wide range of translations. Examples from Whitlam:
Você deve entregar essa carteira à policia.
You should/must hand that wallet to the police.
Vocês não deveriam/deviam estar aqui.
You shouldn't be here.
Não se deve mexer em direitos adquiridos.
One should not meddle with established rights.
O Presidente deve fazer uma declaração amanhã.
The President is due to make a declaration tomorrow.
O preço do petróleo não deve cair tão cedo.
The price of oil is not expected to fall any time soon.
Yes, similar to being in a restaurant and saying "Eu quero uma cafe" vs "eu queria uma cafe". The first apparently sounds like a demand (I want) vs a request (I would like).
What confused me at first was that words like "queria" were used instead of "quereria" and "devia" instead of "deveria". The seconds is the correct way and the first is just the informal spoken way, it seems.
It's not present. Eu concordo is present, but "devíamos" is another way to say "deveríamos" (we call it the future of the past or conditional, to use for situations that would/could happen but didn't, or in this case, situations that are a possibility). e.g.: "Eu acho/achei que ela ia/iria na casa da tia" (I think/thought she would go to the aunt's house), for some reason we like to use the imperfect past in place of the future of the past. So, in this case devíamos is not acting like an imperfect past.
Uuh, good question. The same way as in "I agree with you that we should have tried again": "Eu concordo com você que nós devíamos ter tentado de novo" or "Eu concordo com você que nós tínhamos que ter tentado de novo". And yeah, both sentences can mean a possibility that didn't happen or something that really happened.
According to http://www.conjuga-me.net/verbo-dever, deviamos is Pretérito imperfeito. I get that it is maybe colloquially used as a conditional as well, but Duolingo should first teach the standard language and maybe then some colloquialisms in extra lessons or something. This is a bit annoying.
Most Brazilians will use the imperfect rather than the conditional.
The imperfect is easier to pronounce. As an example, a conditional form of "querer" is "querería" and the imperfect form is "queria". It's unusual to hear the conditional in day-to-day Portuguese, even amongst educated people.
Ciberdúvidas explains that the imperfect is preferred over the conditional because of the pronunciation of the extra syllable:
deveria vs devia "Como a vogal da segunda sílaba do condicional é átona, e a consoante seguinte, r, é pouco intensa, vemos que há tendência para a queda destes elementos. Este fenómeno fonético deu lugar a que o pretérito imperfeito tenha tendência para substituir o condicional simples."