wsup bro .. welcome to the forum! if your on 4th gear you wont be able to downshift unless your going 82MPh or lower. the trans wont downshift if itll go straight into vtec. as for downshifting into 3rd gear. around 60MPH.
not sure about the other questions but i do it couple times a day and never had an issue
not sure about the other questions but i do it couple times a day and never had an issue