Since what they have replied to you over email is different from what they have put in JEE 2017 information bulletin, we still do not have a clear answer to the question "if writing all 5 subjects for improvement is really mandatory or not while applying for JEE".
We have sent an email to them asking about this the same day you posted here about the email response. But we haven't yet gotten any response back from the yet.
The forms for improvement-2018 will be out in another 1-2 weeks time. At the same time they might publish JEE 2018 information bulletin too. And there is a very good chance that its going to be very much similar to the 2017 bulletin. And we will still be with no answer to our question. So, try contacting them through email again (this time try with a different email). And if there is any other medium to contact them - try that too. Please do not waste time at this stage expecting reply from them or waiting for the forms to be out. Once the forms are out, there is going to be deadlines for that too.
ALSO - "ask your friends to email them and ask the same question" - when more folks contact them with the same query, there is a good chance to receive some kind of response back from them.
Here is an excerpt from the email that we sent to them last week:
So, try what we have mentioned above. And if you still receive no reply from them, we will be requesting all the members of goiit.com to send an email to them asking the same query. We have a pretty big community of IIT aspirants here at goiit.com - so, together, we should be able to do it. So, let us know how it goes, and we will be here to assist you.