Al-jawab billahi at-taufeeq (the answer with Allah’s guidance)
1) If any Salah, such as Zuhr etc, is missed before embarking on a journey then Qadha of it whilst on the journey must be made in full i.e. four Raka`at.
Qada of Salahs missed whilst on a journey will be two Rak`ahs for Zuhr,`Asr and`Isha.
Regarding Other Salahs on a Journey:
Witr Salah is Wajib. Hence, it must be offered whilst on a journey as well. If it is missed then Qadha must be made.
Allamah Nawawi writes in Sharah (commentary) of Muslim that praying general Nawafil (optional) prayers such as, Salat Ishraq, Dhuha, Awwabeen, Tahajjud etc, is permissible according to all.
Sunnah Salah should not be unduly missed during the journey. However, if the Musafir is in a hurry then, apart from the Sunnah of Fajr, it will be permissible for him to omit the other Sunnah Salahs. If he is not in a hurry nor does he fear his companions leaving him behind then he should offer all the Sunnah Salahs.
(Ahsanul Fataawa Vol.3 Pg. 71)
There is no reduction in the number of Raka`at of Sunnah Salah.
2) Yes, it will be permissible to recite loudly for those Qadha prayers which are Jahri (i.e. Loud recital in Fajar, Maghrib and ‘Isha). However, the Jahri Qadha prayers (i.e. Qadha of Fajar, Maghrib and Isha) must be done during the time of the Jahri prayers (i.e. Fajr, Maghrib and ‘Isha). Also, one has the option to recite it loudly or silently. (Ahsanul Fataawa Vol. 3 Pg. 79)
And Only Allah Ta’ala Knows Best.
Moulana Qamruz Zaman