Regarding InstantSend, regardless of the number of transactions that occur on the network, if I make a transaction, will the confirmation be instantaneous?
Yes, if you use option InstantSend for your transaction it should be confirmed instantaneous (which means you get 5 instant confirmations on your transaction and at the same time your transaction is locked against double spending).
The sixth confirmation will be through proof of work and will finalize the transaction in your GUI wallet.
Dash recentely did a stress test with a lot of transactions and that did not negatively effect Dash transaction fees or Dash ability to process transactions (including InstantSend transactions).
Link :
https://www.dashforcenews.com/dash-stress-test-processes-double-bitcoins-on-chain-transactions-without-issue/If for some reason your InstandSend transaction does not confirm instantaneous, it will automatically fall back to confirmation through proof of work.
Let us know of the outcome if you decide to try it.