Was what we did correct regarding disposing of 10% of the inherited telecom stock profits, and how should we act to sell the shares given their rising price and our lack of knowledge of the principal?

If the shares are mixed (involving a forbidden activity or usurious dealings), they must be sold and disposed of. Purification is not sufficient, because the sin encompasses the dealing itself, not just the profit. Profits realized before you became aware of the prohibition are lawful. However, anything that came after that must be disposed of. An increase in the share's value does not harm; the shares are sold at their current value, and you dispose of the forbidden percentage of the profit after you become aware of the prohibition.