I am in constant contact with the HR who is also trying to make futile attempts of reaching them. The job position is still listed in their website. What should I make of this?
Your inability to contact them is disconcerting but since you're in communication with HR I'd let them deal with it.
Beyond that... in all likelihood, only one person on the team - the manager - will be answer your questions about equipment, software, etc. If he's out on vacation or otherwise sick then I wouldn't expect other employees to jump in. Like maybe the manager is the only person on the team with a credit card and he needs to use it to purchase the equipment for you. Sure, other employees on the team could tell you that, but they might see that as them stepping out of line.
Worst case: you're unable to commence work on your start date and then you get fired for cause. If the United States there are two types of employment - W-2 employees and 1099-NEC employees. The latter are often called contractors. If you're a 1099-NEC employee then there's really nothing you can do. If you're a W-2 employee your options, at that point, would be: you could file for unemployment and / or sue for wrongful termination.
With regard to your having other offers... pursuing those is totally your call. I guess if I were in your position I'd punt on those other offers, asking if I could give them my final answer a week from now and I'd then just bide my time and wait and see what happens with the company that I have the contract with. Unless one of those other offers is for better pay and for a position that you think you'd be genuinely more happy with?