I searched and confirmed these on with devs:

- Addresses rot due to Winternitz way of working. When you send a transaction you re-use the address and this diminishes the security of the previously created address because of the way Winternitz cryptography is based and that has to do with having the ability to be quantum resistant
- Threshold is due to the fact that the attack could blacklist peers. It is the same idea for PoW'ed blockchains that allow for blacklisting of peers. Actually, the 50% on bitcoin is actually less, it's about 40% if the attack blacklists peers. In the tangle that number is 34%
- The Coordinator is not there because the network is not sufficiently diversified but because the network must be big enough to make 34% attacks become impossible in terms of computational power

It is new tech, new explanations and a lot to learn still, yes.
Cheers, hope I helped
@bitworkers