Do not return samples to the batch.
Risk of infection is very high. Sacrificing this small amount of wort makes life easier and give peace of mind.
- sample tubes are difficult to clean. Many are two part and need the base removed to clean properly, and sometimes take effort to reseal.
- samples often need to sit awhile to get to a good temp and to degas. All the while exposed to contaminates.
- hydrometer is delicate so typically we only rinse them
- your sampler does need sanitized. But whether you use a baster or a thief, sanitizing the inside is very difficult. But this isn't an issue when wort only travels into it.
Personally I use large 50ml pipet, which is glass and can be sterilized by heat.
Also I use the plastic tube that hydro meters come in for testing. They use about 1/4 the wort as a two piece tube. So sample sizes are much smaller and no significant volume is missed from the batch, even with several samples.