The restricted (foreign-based) US pilot certificate will be reissued if you apply for the addition of an instrument rating. This type of certificate requires you to carry the associated foreign licence and medical certificate, if applicable. 14 CFR 61.75(g).
"... find someone who helps me flying 10 hours in simulated IFR... "
You will be helping yourself to fly these ten hours of instrument flight time. That is not the responsibility of the safety pilot. Otherwise, you're right it's the IEP knowledge test for the rating gained under the EU-US conversion agreement. The FAA is quite strict about the particular knowledge test to be taken.
The IPC in both cases is identical. The only distinction is that the IPC must be done prior to applying for a standard US instrument rating under the EU-US conversion agreement. A US instrument flight instructor (CFI-I or CFII or CFI-IA, however you want to style it) is considered an examiner for this purpose.
Unless you do acclimatization (training) under an M-1 visa, you do not need a flight school. There are loads of mentors on NAFI/EAA/SAFE who will happily assist as a safety pilot. Or ask around in social groups like Flights Above.