Call continuity between CDMA and WCDMA is not strictly necessary. It will be great if it can be continued from one network to the other, but lets' face it, both voice networks will be phased out in favor of VoLTE.
There will be plenty of PCS spectrum for not just 1 but 2 or 3 WCDMA PCS channels on the combined network. I would like to see the layout of T-Mobile/Sprint holdings in the PCS band. HSPA+42 is no longer necessary since the advent of LTE.
Sprint is already going to use 700MHz for roaming purposes. They will bid on 600MHz but by all indications they will bypass AWS-3.
Which brings up what is the Feds's great fear that if Sprint and T-Mobile combine, the combined company will not bid on AWS-3, thereby reducing the auction proceeds considerably.
Sprint is paying for T-Mobile's customers and spectrum, not their network. I am sure certain network elements will be reused, certain T-Mobile sites will become NV sites.