Is it permissible for me to speak on the phone with a young man or meet him in a public place with Islamic safeguards, knowing that he wants to propose to me, and he refuses to send his sister to get to know me? And would my refusal to do so be considered extremism?

If it is highly probable that the young man is serious in seeking marriage, it is permissible to speak with him to the extent of need and to see him in a public place or see his picture, while observing the Shariah controls such as avoiding seclusion (khalwa), ensuring the absence of temptation (fitna), and limiting speech to what is necessary. If he is not serious, the relationship with him must be cut off. The preferable course of action, to close the door to temptation, is to limit interaction to his proposal to the family and seeing her in her home.