Stopped-alarm-under-voltage generator is in for about 5yrs w when it goes on for the test run it goes off after few min.only instead of the full 10min.

Problem with this, guessing that normally the power generated is somehow dumped when there is no real load applied.

Digital voltage regulator with ±1.5% no-load to full-load RMS regulation.

https://www.google.com/search?q=20+RESA%2C+testing+with+no+load

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The two alarms simultaneous is questionable. My first thought is faulty control board but requires further troubleshooting. Check for loose cable connections and grounds.

As a test; check oil level. If oil level is safe, disconnect oil level sensor and see if problem goes away. If problem reoccurs, troubleshoot oil warning circuit.

Around 15 + or - a little. This indicates the alternator is charging.

See if reducing the number of things connected to the generator will stop your problem. Disconnect or turn off some things that are not totally necessary.
To properly generate electricity, the engine is required to maintain an rpm within a specific range. If you load down the generator with too many things pulling power, the engine will not have enough HP to turn the generator within the set rpm range. It goes under-speed or under-voltage and you get a fault.
Going to the extreme, if you disconnect everything from the generator and it still fails, you have a generator problem

gen sets have to be run at set rpms so that the voltage and cycles are correct for the applications
with the rpms as little as 100 rpms below requirement , the voltage will be down , causing a brown out which burns out electric motors and if the rpms are low the required cycles will also be off which affects electronic equipment
service the gen set and find out why the rpms are not at the set level

When you get the under speed alarm is the generator under load? Try manually starting the generator with no load and let it run for a while, if no alarm, then put it under your home load and measure current draw (maybe have an electrician check it) if the amp draw and in rush caused by compressors, pumps motors etc are within the generator rating, check your fuel pressure, on average 8-11 WC Nat gas. Pressure may test well under no load but drop starving the engine of fuel unde rload. Underspeed is just that engine cannot make RPM/Frequency

check the voltage regulator

It sounds like you are not producing any voltage. When you start your generator have a volt meter on the generator breaker to see if there is voltage present when the genset is running. Also, check you oil pressure switch, which is near the oil filter.

Same problem here but with a Toyota Sienna '99. It took $600 in diagnostic work but the problem was a low voltage electrical short in a switch that was part of the passenger door lock. Specialty shop found it by placing voltage testing equipment on 4 to 6 wires at a time that ran into the cars computer. They recorded any voltage spikes over all these wires 24hrs/day over 3 days to find the bad switch. Snipping the wire to that switch meant the alarm would always go off if the locked car was opened with a key in the passenger door lock but we we're ok with it. Car was about 5yrs. old when this ocurred but we never had another problem of any kind with it for another 4 years. Thanks, Dan

Ive had a look at the manufacturer web site but ther doesn't seem to be any driver for your model. You should be able to turn the battery warning alarm off in the management settings