Is a single photon emitted as a spherical EM wavefront?
No. The photon in present day particle physics theories, i.e. mathematical models which fit the data, is one of the elementary point particles. When measured it has just an energy=h*nu, where nu is the frequency of the classical electromagnetic wave that will emerge from a large number of such photons.
This is clearly seen in the one photon at a time recording of the two slit experiment:
Single-photon camera recording of photons from a double slit illuminated by very weak laser light. Left to right: single frame, superposition of 200, 1’000, and 500’000 frames.
The frame on the left shows individual photon hits at an (x,y) as dots. No spherical wave. The wave nature appears with the accumulation of data, i.e in the probability distribution of finding a photon at (x,y). Quantum mechanics is all about probability distributions.
If not, how does the photon acquire the wave-ness if it is not born as a spherical wave? Also, in second case, how can multiple photons synchronize to make up a single wave front?
The probability distribution shown is the complex conjugate squared of the wavefunction describing the single photon. The wave function is a solution of a quantized maxwell's equation, and has in its complex function the electric and magnetic fields. Since there is continuity (use of Maxwell's equation) between the classical regime and the quantum mechanical it should not be surprising that the variables measured in the classical regime are carried in the wavefunction of the photon in the quantum mechanical regime.
When there are many photons the classical field emerges by the superposition of all the photons wavefunctions. This has been treated mathematically in quantum field theory in this link.
See also this answer of mine on a similar question.