The party whip is considered to have the ability to cast all votes for their party without a specific proxy from individual members, and if a member wants to vote differently from their party then they would instruct that whip that way, although in that instance they'd be quite likely to go to the chamber and cast their own vote.
The reasons for their absence and the 25% allowance of members who are able to be away from the parliamentary precinct is a matter between the whip and the members of their party. There are not specific reasons provided for in Standing Orders for those matters. Really it is whether the whip will permit them to go, and so they do have to convince the whip. When I talked to the government whip he said everybody except the Prime Minister has to give him reasons for being away. Those reasons are really up to him, and I think it's so that he can weigh up competing requests for absence.