SOLUTION:
I just found a big big post: Re-commissioning N9 in 2018
Following its step, I finally installed the N9QTweak package and installed the aegis-dpkg command. With that I was able to install my the bejeweled2 deb file.
I am still not 100% about openmode, but installation log said that it was succesful, so let's hope it was.
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I have flashed the openmode kernel, but aegis is still active

Here is what I did:
1. Get rid of the warning:
$ disclaimer-cal remove View-openmode
disclaimer-cal: Disclaimer removed from CAL (truncated to zero size)

The flashed my phone like this: (using windows 7)
flasher -f -F main.bin -F emmc.bin -R && flasher -a main.bin -k zImage_2.6.32.54-openmode_l2fix --flash-only=kernel -f -R

The result of that:

Found device RM-696, hardware revision 1507
NOLO version 2.3.6
Version of 'sw-release': DFL61_HARMATTAN_40.2012.21-3.339.2_PR_339
Sending ape-algo image (7103 kB)...
100% (7103 of 7103 kB, avg. 32583 kB/s)
Suitable USB interface (phonet) not found, waiting...
Ping attempt 1 (250 ms)
Server application: 1.7.2
Found product RM-696 rev. 1507
Server implements softupd protocol version 1.8
Image SW version DFL61_HARMATTAN_40.2012.21-3.339.2_PR_339
Image moslo not present
Image tar skipped
Image config skipped

Battery level 86 %, continuing.

image [state progress transfer flash speed]
---------------------------------------------------------------------
[x] cert-sw [finished 100 % 1 / 1 kB NA ]
[x] cmt-2nd [finished 100 % 95 / 95 kB NA ]
[x] cmt-algo [finished 100 % 789 / 789 kB NA ]
[x] cmt-mcusw [finished 100 % 6050 / 6050 kB 3142 kB/s]
[x] xloader [finished 100 % 23 / 23 kB NA ]
[x] secondary [finished 100 % 94 / 94 kB NA ]
[x] kernel [finished 100 % 2714 / 2714 kB 1927 kB/s]
[x] rootfs [finished 100 % 1170221 / 1170221 kB 10693 kB/s]
[x] mmc [finished 100 % 1627219 / 1627219 kB 16753 kB/s]
Updating SW release
Success
flasher 3.12.1 (Oct 6 2011) Harmattan
WARNING: This tool is intended for professional use only. Using it may result
in permanently damaging your device or losing the warranty.

Picked ape-algo from a FIASCO file
Suitable USB interface (bootloader/phonet) not found, waiting...
Found device RM-696, hardware revision 1507
NOLO version 2.3.6
Version of 'sw-release': DFL61_HARMATTAN_40.2012.21-3.339.2_PR_339
Sending ape-algo image (7103 kB)...
100% (7103 of 7103 kB, avg. 32287 kB/s)
Suitable USB interface (phonet) not found, waiting...
Ping attempt 1 (250 ms)
Server application: 1.7.2
Found product RM-696 rev. 1507
Server implements softupd protocol version 1.8
Image SW version <none>
Image cert-sw not present
Image cmt-2nd skipped
Image cmt-algo skipped
Image cmt-mcusw skipped
Image xloader skipped
Image secondary skipped
Image moslo skipped
Image rootfs skipped
Image mmc skipped
Image tar skipped
Image config skipped

Battery level 90 %, continuing.

image [state progress transfer flash speed]
---------------------------------------------------------------------
[x] kernel [finished 100 % 2712 / 2712 kB 2068 kB/s]
Updating SW release
Success

I also installed n9repomirror and enabled developer mode successfully.

Then I tried to install bejeweled2-full_0.0.60_armel.deb from a game repository and this happened:

/home/user/MyDocs # dpkg -i bejeweled2-full_0.0.60_armel.deb
x509_sigverify: Certificate failure reason: certificate has expired
x509_sigverify: Certificate verification failed
Aegis rejecting bejeweled2-full_0.0.60_armel.deb: package 'ea-mobile-bejeweled2-full' origin cannot be determined -- signature check failed
aegis aborting dpkg -- all listed package files rejected
Compilation failed in require.

So I do not understand what went wrong!
I thought the openmode kernel disables the aegis security system on the phone.
Is there any other way to check if the openmode kernel is active?
It does not seem to work. :-(