BikeRider22

Dear Experts,

I'm in the process of upgrading an old road bike's cluster
from Shimano 8 speed to 9 speed.

So far, I have 9 speed
- front cranks
- rear cluster

What I'm wondering is which
downtube shifters can I use.

I have a set of Shimano SL-7900 downtube shifters.

q1:
If I get a 9 speed Shimano rear derailleur (say Altus),
will the SL-7900 downtube shifters work with with 7900 DT shifters?

...

Now, I also have a 10 speed rear derailleur (105)

q2:

Could I use the 10 speed downtube shifters and 105 rear derailleur
with the 9 speed cassette?

Thanks a lot!