Gas Tubes: neutralization of space-charge causes drop in tube resistance? Deionizing voltage re-combination?
Regarding the first question, the reason is that the negatively charged space charge itself creates an electric field, which is in opposition to the electric field generated by the positive anode voltage. Increasing the number of negative charge carriers available for conduction is offset by the increase in the opposition electric field these charge carriers generate. 

This puts a theoretical limit on the maximum anode current density for a given anode voltage when a space charge is present, and for a planar plate geometry is given by the Langmuir-Child equation:

$$1.67×10^{-3}\left(\frac{q}{mc^2}\right)\frac{V_a^{\frac{3}{2}}}{d^2} \frac{A}{m^2} $$

Where \$d\$ is the distance between the plates, \$V\$ is the anode voltage, \$q\$ is the particle charge, \$m\$ is the mass, and \$c\$ is the speed of light (all SI units).