Can a player be replaced after the team is announced for a cricket match?

Once the team has been named and exchanged the players can not be replaced without the agreement of the other captain.

If the captain allows then the player can be a complete replacment and has no limits. This happens very infrequently.

More common is a substitute, a substitute can be approved by the umpires for any injury, worsening of existing injury or exceptional circumstances.

A substitute, may not bat, bowl or captain the team. In the 2017 laws it was changed to allow the substitute to keep wicket.

If a batsmen needs a substitute they can have a runner to run for them but this has to be a player who is nominated and they should have batted.

There has been a new playing condition added for international cricket that allows concussion replacements (https://www.timesnownews.com/sports/cricket/article/what-is-concussion-substitute-rule-heres-everything-on-iccs-new-set-of-guidelines/471184) first used for Steve Smith in the 2nd Ashes test 2019.

Players can be replaced if they got injured otherwise after specific time they can not be replaced.

To answer of your second point, most players can be replaced if they are injured. But during the match you cannot replace the players because it is against the cricket rule.

After the nomination, player replacement shall only be allowed with the consent of opposing captain.Once done replaced player can do his full duties as a nominated player.

After nomination a player can be replaced at any time during the game.But it has to be with the consent of the opposing captain.