, your sources are not quite correct or you might have misinterpreted them. Or perhaps you accidentially accessed documents from a smear campaign.
He was a co-founder of the umkhonto we sizwe, however at that time they restricted themselves to sabotage acts against the racist government. The terrorist acts you mentioned happened later, when Mr Mandela had already distanced himself from them.
Before that he had worked as a lawyer and specialized in defending victims of racism and working against the apartheid government, for which said government kept trying to get him into prison but for a long time failed to construct a watertight case against him. They only succeeded in 1964 and he was sentenced to lifelong imprisonment for sabotage. In his final speech at court, when he was facing the possibility of being executed, he did indeed speak of the necessity of an armed fight agaist the government since peaceful means had not worked. But that was not meant to make people start terrorist acts but rather as a defense of a possible civil war (like the Namibian liberation war. Namibia was under South African occupation back then and around 1964 the resistance went up to a warlike dimension that lasted till the late 80s).
If you check the dates you cited you will see that Mandela can impossibly have participated in the terrorist attacks. From 1964 on he was continually imprisoned in different locations, mainly on Robben Island.
In April 1982 he was transferred to Pollsmoor Prison in Capetown where he remained till December 1988
After he got TBC there he was moved to Paarl where he stayed imprisoned till Feb., 11th 1990.
You will admit that - particularly in as tightly controlled an environment as ZAn prisons were back then - one can not throw a bomb on people or orchestrate terrorist acts and executions without trial while being imprisoned hundreds of miles away.
I wholeheartedly agree that those deeds of the MK were absolutely inexcusable, but we must not blame a parent for the acts of an evil child.
The fact that Mr Mandela received the Nobel Price for Peace supports his relative innocence. You don't get that price with a terrorist past.
(edit to add my sources:
For the 1964 speech in court and the time of his imprisonment see any bio of Mandela, for example the one on Wikipedia.
Source for info on Namibian liberation war: myself. Been there during the mid-80s in a development aid project near Walfish Bay)