The advice you have received seems very odd - and to advise someone to seriously amend their lifestyle as you were asked to do is in my book "crossing the line".
Try to continue with the IVA if you feel that you will be able to keep making the payments - after all it is set up now and the end is in sight, however with the antipated salary reduction this may not be possible. It may, however, be feasible for you to apply to your creditors for a variation of the terms, and thus make lower payments. This does not always mean an extension of time, and you should discuss this with your Supervisor.
As you appear to have lost trust in your current IP firm, it might be sensible to take a second opinion if you feel that you are not getting the right level of advice.
Regards, Melanie Giles, Insolvency Practitioner for over 20 years.
For further details contact me at
http://www.melaniegiles.com and view my IVA blog at:
http://melaniegiles.blogs.iva.co.uk