These mihrabs are nothing but niche(s) sans Vigraha as suggested by E. B. Havell.
1. Trefoil arch resting pillars with Kalasha base(such pillars are depicted in Shunga sculptures)
2. Trefoil arch with extant Shikhara of Kalinga type
Note the proportions of trefoil arch and columns. Exact same and why not, after all both are product of same tradition. It is a lie that Torana were only wooden gateways of Sanchi type.