Attiny861 programming current
Would here be some problems if I connect two voltage sources without any protection - the internal ldo and an external source? With the same voltage of course.

That's a bad idea, doing it correctly gets quite complex.
What you should do is have a suitable power supply, a common ground, and possibly gating circuitry (sometimes a variation of a level shifter) to make sure you never violate the rule against applying I/O signals to a device which is unpowered (unless the data sheet explicitly permits that, which that of an ATtiny most definitley does not).
The 10 mA LDO limit seems unusual; if you are using the on-chip LDO of a USB-UART or similar, the real answer is not to do so.  Given you are &quot;programming&quot; you probably have a USB connection in there somewhere, so get an ordinary 3v3 LDO of a healthy hundred milliamps or so rating, the necessary bypass capacitors, and wired it up to run off the USB V-BUS. An LDO with its own overcurrent shutdown wouldn't be a terrible idea, either.
That said, my guess is that when running at slow clock the current programming your MCU won't be much over 10 mA if even that much.  By why cut it close?