You ever noticed how brass doesn't stay shiny very long? When you remove the tin plating on the pins, the brass will oxidize more quickly. If you have to for the short term, use one of those small brass wire brushes which is less aggressive.

With the machine OFF, set your multi-meter to low resistance mode.
Measure the resistance across a working control lamp socket - you will read in the vicinity of 3 ohms.
Then measure the resistance across those two suspect lamp sockets. If you read zero ohms there is a short in the socket/lamp. If they read the same as other working lamp sockets then you will need to trace back the wires on those two lamps back to the lamp driver board to see where they are connected in case someone's put the wires in the wrong location.

Are the 2k bonus, 4k bonus and 7k bonus lamps working in lamp test mode?

Take the 7k lamp out and measure across the socket again. If it's still 1.5 ohms the socket has a short. If it's open circuit, install a new lamp and remeasure looking for approx 4 ohms.

BTW when you touch both meter leads together in resistance mode what reading do you get?

So that lamp/socket are working you need to go upstream to the lamp driver board.

So ground the anode leg of SCR Q59 (look at the reference links below for SCR leg definitions). If the "B Saucer" lamp illuminates, connection from the SCR to the lamp is ok. If not you have a bad connection somewhere between the lamp board and the lamp.

The methodical approach is:
(1). Ground the tab at the lamp socket. This completes the ground power path to the lamp and bypasses the logic boards. If the lamp illuminates the lamp/socket are ok. If it doesn't illuminate, inspect the lamp/socket.

(2). If point (1) works, then go upstream to the lamp driver board. The anode leg of the associated SCR connects to the tab wire at the lamp socket, so grounding the anode SCR leg completes the ground power path to the lamp. If the respective lamp illuminates, you have good connection from the SCR to the tab wire at the lamp socket. If it doesn't illuminate, inspect the respective crimp terminal at the lamp driver board connector for that lamp and check its pin header soldering for cracks.

(3). Finally if point (2) works, then you can manually switch on the SCR to test the SCR functionality. Connect a jumper wire at TP3 on the lamp driver board. Connect the other end of the jumper wire to the "gate" leg of the SCR. This electronically switches the SCR on. If the lamp doesn't illuminate the SCR is faulty. If it does illuminate then there are other components on the lamp board to investigate, but they are rare failures.

Reference links to the SCR leg definitions:
https://pinside.com/pinball/forum/topic/stern-trident-feature-lights-quit-working#post-4980816
https://pinside.com/pinball/forum/topic/stern-trident-feature-lights-quit-working#post-4981037

Yep pretty much.
Confirm the process by jumpering TP3 to the gate leg of another similar working SCR and that it's associated lamp illuminates.
Replace SCR Q59 when you are happy the process has proved Q59 is faulty.