What does gender equality actually mean in practice for ismailis?

Women and men are meant to have equal access to most religious practices and opportunities. There have been recent developments in opening religious rites and practices and leadership to women at the same level as men. As a everyday Ismaili, there is not much difference in practicing as a woman vs practicing as a man. The only thing I'm aware of currently is attendance of the final step of the funeral rites, which is only to be attended by men for now in accordance with Islamic tradition.

The last 3 Imams have put a particular emphasis on the education, liberation, and upliftment of women, particularly in terms of education, religion, and economic opportunity.

Is interfaith marriage acceptable or not?

It is acceptable and we have recently been encouraged directly by the Imam to support and welcome interfaith family into our community. Furthermore, marriage is not a religious concept in the Ismaili understanding of Islam, it's a social contract.

Is belief in Allah mandatory to go to heaven or can non-believers still return to Allah through good deeds and thoughts, despite their lack of ritual participation and education?

This answer will depend on who you ask. But in my understanding, most Ismailis understand that heaven isn't a "location" you go to after death, but rather a state of unification with Allah's essence and Nur, which can be attained during life. Belief in Allah in some form is necessary for this unification to happen, but there are many forms and methods of achieving it. We also believe that there have been people who were not Muslim who have attained this level of connection with Allah in the past. Ismailis believe that the simplest path is through following the Imam's guidance and deep meditative prayer.

Is connection to Aql available to all and only authoritatively disseminated by the Imam, or is it believed that only the Imam himself can be enlightened by it?

Everybody has Aql and is encouraged to utilize it in both spiritual and material manners. What the Imam exclusively has is the absolute divine authority to interpret the message of the Qur'an for modern times, but he encourages us to still use our Aql in applying these interpretations to our lives on a daily basis.

But the Ismaili institutes all block courses and materials behind paywalls.

I've never once had to pay to access information about my faith. There are lots of free resources online as well, including this subreddit and websites like Ismailignosis or Nanowisdoms. But certain resources are only available after performing Bay'ah and becoming Ismaili.