Difficulty is adjusted approximately every two weeks, so the network would self adjust to a reduced mining rate.
Mining isn’t used to validate transactions. You can validate transactions without mining. It’s purpose is to secure the network. The network validates transactions using asynchronous cryptography and nodes only transmit valid transactions. Mining secures those transactions from being overwritten.
If your crypto-coin starts life with all the coins instantly sprung into existence, that means everybody gets to buy their coins from you--the founder. There is a name for this--it is a pre-mine scam. All you do is mine most of the coins yourself, then go pimp your coin so it gets some buyers, and sell them all off for a sweet profit.
It isn't that new coins can't be created, a coin may be worth e.g. $100 and it costs $101 in electricity to create another one so nobody does until the demand for coins increases.
But the amount of work required to validate a transaction need not be as much as it takes to create a coin, and people will continue to be willing to pay someone 5c to do 4c worth of computation to validate their transaction.
Yep, a non deflationary currency would be precisely one that has its coins created all the time, instead of in a predefined amount as bitcoins. (You can think of mining as "discovering" one of the predefined bitcoins instead of creating a new one).
The problem is that for a currency to be slightly inflationary, as it should be, you need to tie it to an economy and make political decisions about it- and then you're back at square one.
Mining isn’t used to validate transactions. You can validate transactions without mining. It’s purpose is to secure the network. The network validates transactions using asynchronous cryptography and nodes only transmit valid transactions. Mining secures those transactions from being overwritten.