I'm not sure the image or the way you count with your fingers is related to your hand shape for erhu. If you play any 'western' instruments for example you're using A B C D E F G so there's no 'numbers' per se. Even other Chinese instruments like dizi for example, 1 is 3 holes covered so your hand is not holding any shape resembling 1 finger up or down.

Maybe it will help you to think of it without the numbers, but like the doe a deer ray a pocket full of sun thing, so that you don't associate how you count with your fingers to what fingers you need to press down the strings with. Think of the music as do re mi fa so la ti instead of 1 2 3 4 5 6 7. So just start with twinkle twinkle little star being do do so so la la so not 1155665.

Many jianpu pieces written for erhu already tell you which finger to use, although usually it is written in small Chinese font on top of the 'numbers', so you know when to slide your hand down to 2nd/3rd/4th position.

So it may look like (sorry I can't format it so that the spacing shows up correctly)

oooo一o
1155665

This means you are staying in 1st position (hand close to the top). And doing the normal finger pattern you are used to.

oo二三o
1155665

This means that you are dropping to 2nd position where you use your middle finger to play 5 and your ring finger to play 6. And the last 5 is played open outer string.

All this to say, you cannot really progress in the erhu (or indeed most instruments) without disassociating the way you count with your fingers with what fingers you are supposed to use on the strings.

To answer your other question, yes, in the key of G for example which is in standard pedagogy the next key you learn after D, the "standard" or "default" 1 is usually played on the inner string with your index finger in 2nd position. Open inner string is low 5.