Those injector tests confirms that the audio section is working, as you already know.
You need to use the tracer function of the Velleman K7000 to check for AM signals.
With your tracer probe on the centre tag of the volume control, do you get tunable AM stations? You should do. If you don't hear AM signals at the centre tag, could you have disturbed the wiring around the rear wafer (S2 on the KB Service Sheet)? Check for a signal from the 2nd IF where R14 (100k) on the KB Service Sheet joins point 2&3 on the rear switch wafer. If there's a signal there, but not at the volume control, that suggests problems around the 'switchery'.
I must say that I find the unconventional style of the KB circuit hard to follow as compared to the Trader Sheet.
On the Trader Sheet rather than the KB circuit, you can see more clearly that V4a - the AM detector diode, rectifies the IF signal, picking off the AM signal which should appear at the junction of R16/R17, before R17 goes to the switch wafer, S12. If you don't hear AM signals there, that suggests that the AM detector isn't working. If you hear AM signals at the junction of R16/17 (Trader Sheet!), but not at the volume control, that could point to a problem with S12.
I think signal tracing give you the best chance of finding where the AM signal is being lost. The best tutorial I've come across is at the link below. It's a circuit of a conventional 'AA5' ('All American five tube') AM superhet tube/valve radio. The instructions tell you to click on the circuit, then hover your cursor at different points - A, B, C etc. Click on any of those points and text will appear to tell you what you should hear (or not hear in some cases) from your signal tracer at each point, and if you don't, it suggests where the fault may lie with that stage of the radio:
http://www.nostalgiaair.org/Referenc...ost/post01.htm
Given that the set was working fine till you change the 10Meg resistor, you need to discover what you've disturbed to cause the AM section to stop working. Could you for example have inadvertently caused any wires to short out in the scary rats' nest of wiring around the valve-holder? (Which I only view from behind the sofa!
These are just some 'off the top' thoughts, but I'm not at the top of the food chain so others may have some better ideas.
None of this makes any sense of course without both the KB circuit and Trader Sheet 1233 in front of you.
Hope it might help a bit.
Good luck with it!