Those are fair points. My initial thoughts:

1. I don't see any feasible way to do this other than voting online. Because digital-only voting is inherently insecure, I envision this as a preliminary step. If enough people vote to cross the minimum participation threshold, and their votes would change the outcome of the vote, a special election/vote could be held. For less time sensitive votes, bundle them up and hold a special vote for 3-12 months' worth of legislation. More time sensitive votes could pose more of an issue though.
2. This one would take some thought to solve. Voters are likely to get fatigued if they are asked to vote on every version of every bill. I guess I envision this as more of a way to ensure specific legislation does or doesn't pass if the people have a very strong opinion. Maybe only take the popular vote into account after the dust has settled, allowing citizens to repeal a law within a set period of time, or resurrect a shelved bill that failed to pass one house or the other.
3. Yeah, this could be a problem. Lopsidedly mobilizing particularly large amounts of constituents could flip the vote against popular opinion. This could be mitigated somewhat by requiring a minimum percentage of the constituency to vote, and by holding a formal vote to verify authenticity.
4. This could pose an issue either way. If we use digital voting for initial screening, that could under represent the older, generally less tech-savvy generation. Starting with a more time-consuming formal vote could over-represent them.

Your fourth point led me to another issue too--we would be taking the popular vote into account to skew the Congressional vote. Would this skew the vote unfairly? Could it give unfair power to a very dedicated, politically-aligned minority?

Suppose in this system we have strong voter suppression and apathy. Suppose 40% of constituents form a voting bloc that elects a minority party that has perhaps 40-45% of the seats in Congress. By combining the popular vote with a congressional vote as described above, it would be possible for the minority party to pass legislation without the majority party having any say. We could end up with minority rule.

Perhaps tweaking the rules a bit would help? Instead of citizens directly voting, perhaps each state forms a voting bloc, and the popular vote can override politicians' votes. Going state by state presents additional feasibility challenges though, in that it would either require an even more dramatic change in our voting system or would dramatically slow down proceedings when senators' votes are disputed.

---

Thanks for the feedback. It seems this concept would need a lot more consideration, but it still might have merit if these weak points can be addressed