I looked at the schematic for the RoadKingII, the RKII footswitch may work but it has a few extra features the Roadster does not. Not sure it will use the same cable either. Probably not an option. However, the Roadster does use the same preamp as that of the RoadKingII. The only difference in the schematic is the power output section, progressive linkage, and two fx loops. Obviously, it is not the same physical preamp board though. Just the circuit topography and design is the same.
As for where the gain control is located for CH2, it sits between V1A and V1B as it is part of the tone stack driver. Both CH1 and CH2 have the tone stack driven from the plate of V1A. CH3 and CH4 also have the gain controls in the same location but the tone stack is not positioned off of V1A. After V2B triode, the signal path for CH1 and CH2 will enter into V2A which then has the CH1 and CH2 master volume circuit and at that point it passes onto the FX loop circuit.
You could try borrowing V2 from the Mark V90 as that tube only uses one triode. See if using that tube in the Roadster V1 makes a difference. If no change, try it in V2. It is hard to say if the original owner moved tubes around in the amp. However, it could also be the gain pot for CH2 but that I doubt is the issue. The Brit mode does bring in a 100uF cathode bypass cap on V2A triode that may be enough to encourage the tube to saturate too much if that tube has been degraded or near end of life. It is possible the previous owner swapped V5 with V2 since the cathode follower circuit used in V5 is the A triode and V2A is used only with the CH1 and CH2 circuits.
Before going too deep into tubes, the Roadster can use any 12AX7 tube in the following positions: V1, V2, V4, and V6. The two critical tube positions are V3 and V5 of which have cathode follower circuits. Those two positions, either JJECC83S (same as the Mesa 12AX7 tube) or a Chinese 12AX7, or 7025 will work without premature failure issues. Some NOS 12AX7 like a JAN/GE 12AX7WA will also work well with the cathode follower circuits as I have been using those in my Roadster for some time.
There could be another thing to consider but since all of the other channels and modes are working fine, I would assume that CH1 Tweed does not have the gain problem as that is very similar to the Brit but does not use the 100uF cathode bypass cap. To check for the problem I am thinking of will require removal of the chassis from the head shell. It is safe to do this but just keep your fingers from touching the internals. I always pull the chassis out when I do a tube roll. That way I can keep the power tubes in place and not have to wait for them to cool down. Since the Roadster head has the tubes pointing upwards, the tube shields can result in aluminum dust falling down around the tube socket. this is usually caused by many tube swaps by rotating the shields to remove them. The chassis needs to be out of the head shell, shield and tube removed. Look down into the hole with a flashlight to insect for any silver flakes or dust around the tube socket. If it is present, you can use a Q-Tip (not sure what they are called in Europe) dampened with acetone to collect the aluminum dust. Do not use isopropyl alcohol as this is mostly water and will not evaporate completely. It may require a few more Q-tips to get the job done. If you have compressed air available that can also help but get the bulk of the dust cleaned up with the Q-tip first. I had this issue with the Royal Atlantic as I was tube rolling that amp a lot. I also have a tone issue with my Roadster so it is time to check for the dust. If the small back rubber rings on the tube shields are silver in color, yep, that could be the issue. When cleaning up the dust, I will also remove the rubber O-ring and clean them too. They are supposed to be black in color.
Note, if nothing works to cure the issue and the characteristic is still resulting in the scratchy static sounds, it is more than likely the preamp tube but if a new tube does not change that, the last resort is the gain pot. If the gain pot is bad, it may need to be replaced. I usually look for more obvious issues before attempting to change parts that require any soldering. Take into account there could be potential voltage stored in the power supply capacitors high enough to risk severe electric shock. Proper procedure must be done to safely discharge all capacitors in the amp. For now, let's focus more on easy things like tubes or dust.
As for the footswitch: Mesa from what I understand is so far backed up with orders, they are not taking any custom work. I want to get a creme/back just faceplate for the Mark VII since that option was going to take several months to get.
I would send an email to customer support regarding the footswitch, at least find out if this will be available sometime in the future. The website claims: temporarily out of stock. I would assume it is still available.
https://store.mesaboogie.com/products/footswitch-roadster.html
I hope the preamp tube can fix the issue. It you are waiting for the tube to arrive, instead of borrowing the V2 from the Mark V90, you could opt to exchange the tube in V4 of the roadster with V2 in the roadster as that is used on the reverb circuit. You can also try it in V1 to see if that makes a differences. When it comes to used tube amps, it becomes questionable if the preamp tubes have ever been changed or if they were, were they of good quality and new before the amp was sold? Same would apply for the power tubes and Rectifier tubes. You may need to consider some replacement tubes. At least they Roadster and Mark V both use the 5U4GB rectifier. 6L6GC tubes may be the same or different. That just depends on what you like. Roadster does not rely on power tube distortion as much as the Mark V90. As for preamp tubes, as I mentioned above, V3 and V5 are the critical tubes that you need to be concerned about with the Roadster. Any other 12AX7 tube in the other positions are fine, same would apply to the Mark V90, that amp can run any 12AX7 preamp tube as it does not have any cathode follower circuits to be aware of.
