"Alle sind bisher willkommen."
I don't think "All are welcome up to now." is a native English utterance. "up to now" requires "have been," I believe. Same with "so far": "So far all of them have been welcome/welcomed."
I don't think so. "Sind" can be translated with present simple (are) or in some cases with present perfect (have been), but hardly with the past (were).
German often uses the present tense for things that we would say using a past or future tense. For example the question 'How long have you been here?' is 'Wie lange sind Sie schon hier?' or word for word literally 'How long are you already here?'
German often uses present for future, but not for past. "Have been here" is not a past tense in English either, it is present perfect. It tells about something that started in the past and is still going on. In German (and I think in many other languages including my native Russian) you don't care when something started, what is important is that it is still going on, that is why you just use present tense.
"All were welcome up to now" is clearly past. They were welcome but they are not now. This is not what the German sentence means.