As far as starting a war, Obama invaded Libya under the AUMF. He was also the first to launch airstrikes in Yemen and bombed sites in Syria, Somalia, and Pakistan. During Obama's administration, the CIA had launched 330 strikes on Pakistan alone. In comparison, his predecessor, Bush conducted a total of 51 strikes in four years.
But you are correct, while under AUMF President Trump could legally retaliate for attacks from Iran, but to actually declare war would require Congres, which he would not get.
AUMF's do sunset. He can send in additional troops, but only for 90 days and then he would need Congress's approval. During Obama's administration, Congress had extended the AUMF for operations in Iraq and Syria indefinitely. Congress has yet to revoke that authorization.
As far as a draft, I think there is even less likelihood of that. That would require Congress to pass a law to re-initiate conscription. Besides, in today's world, the need for millions and millions of combat troops is unnecessary.
I agree Congress has given up to much power to the President, the courts, and to the bureaucracy. Most regulations no longer come from Congress but rather from the alphabet of departments. The courts almost seem to make up laws rather than to determine the constitutionality of laws.
In my opinion, Congress is shirking its duty. I think Congress did this to stay out of passing laws that people didn't like. This way they have reasonable deniability.