First, it is good the coach talked with you in more detail, and offered some home stretching guidance. Positive points. And as you have already agreed, repeating L2 is not necessarily bad if her training does actually keep her progressing in both technique and skills, and sets her up for a successful season of L2 as well as a jump start on L3 for next year.
The "may be ready to join L3" at the end of summer suggestion is still weak and vague to me, though. Did the coach offer any specifics on how she would be evaluated and when? And what exactly she would need to demonstrate to move into L3? Summer only has a few weeks left. The current L3s should already be perfecting routines. Usually the end of summer is already the first competitions (unless your season is different - I didn't go re-read in case you mentioned that....). So is the coach saying she would start off competing L2 and then switch to competing L3 mid season? I have seen too many 'false hope' suggestions made by coaches to momentarily placate families into thinking there is a chance, but failing to specify actual testable goals and criteria and failing to follow up on these 'suggestions' when the time comes, while the athlete and family thought all along they were working toward an actually realistic goal. That seldom ends well.
You are correct that unless the new L2 coach takes a different approach, the gap between the L3 girls and your daughter will only widen. Again, we have been there. And again, my DD did surpass those girls eventually, as may your DD, but I still regret the mostly wasted year as this was age 8-9 for my daughter, a prime age to be working toward higher skills, rather than spending 9 months perfecting L3 routines that were already scoring solid 9's. Point of reference - she started L3 season in low/mid 36s and ended at mid-high 37s in a state where 36-low 37 wins everything. Compare to her group that started off way back skill and form-wise, largely starting off season at 31-34, some scratching events, ending at 35-37, you can see where the focus was and why they made way more progress while my DD just maintained. A few tenths are nice, sure, but over 9 months that was all she had to show for it - a few tenths and no new skills for a talented athlete at a prime age.
So proceed with caution. Maybe the new L2 coach is awesome, her flexibility and form improve, and you survive another year of L2 and get the great L3 coach next year, and all is happy. If that sits well with you, then help your daughter feel good about it, relax, and enjoy her young super-adorableness at meets.
But I would still try to get more detail on the possible move up from the coach if you have not already done so to avoid a 'false hope' situation. Pardon my skepticism - I have just seen it sooooooo much.