Regarding GST Tax

Yes. once the invoice reflected in your 2 A you are eligible for that input

You can always have or avail the credit for the ITC reflected in your GSTR 2A , unless untill it's cover under section 17(5).

If you have valid copy of Invoice which prescribed u/s 31 R.W.R 46 then you can claim the credit.

There are four condition under section 16
1. Possession of tax invoice
2. Received goods or service
3. Tax paid to government
4. Return must be filed

So if u have valid Document and its reflecting in GSTR 2A you can claim ITC

Yes there might be problem in availing credit for those invoices whose supplier has not filed GSTR 3B , As per the provision of Section 16 , one of condition to avail ITC is that Supplier has Deposited the Tax & As per Notification 49/2019 CT , GSTR 3B has been inserted as regular return in place GSTR 3.

So , the Tax Officer can ask for reversal of ITC for Such Invoices.