You can use DIGU but you would have to set MMTTY for reverse tones because you would be on the opposite side band to everyone else.
You can use SSB - LSB for normal tones and USB for reverse tones. You would not have the automatic control over your signal gain that the DIG mode affords (hence its existence and recommendation for use). But if you conscientiously limit gain to prevent over-driving, splatter and other anti-social behaviour such as using compression, then yes you can use SSB if you want to.
The answer to your last question is: yes. But to rise above the minority you see in RTTY contests calling CQ with no replies because they are on the wrong sideband, or transmitting artifacts across the spectrum because of over-driving, the recommendation I have is to follow the Addendum. It is not a rule book but is sound operating advice.
There is also a useful convenience pertaining to this for ESDR2 users who operate both RTTY and JT65/FT8/FT4. With DIGL correctly used for RTTY, set as it has to be with the necessary offset, one can then devote DIGU to the other digital modes where the offset must be zero.
