Well, your Apollo only has two inputs, if you want all 8 tracks coming out of the tape machine, then you will need 8 inputs or a mix down.
Your analog coming in dry is pre-processing. The reason for this is, if you set compression, then the compression inside the DAW can be adjusted and your dry signal is not changed. Only when it is sent to the master bus is it compressed. Same thing with dry guitar input.
Unprocessed signal you can adjust the effect, reverb, EQ, whatever and if you don't like it, you can adjust it without having to do a new take because of a bad choice your effect's setting.
This isn't that uncommon from the analog world. Especially after multi-track recorders. Dry input to the channel, that recorded channel was then ran back out to a post processor and then the post processed signal would then be recorded to a new channel. But the original take would not have to be done again.
In digital, post processing signal is done "in the box" and the effect is heard, but it doesn't change the original track recording. Much more forgiving.
One great take and now you can change amps, EQ, compression, whatever to optimize for the mix.
If you want to use outboard gear, then I think it has been stated, record, bring it into the DAW, run it back out to your hardware and back in to a new channel that is post processing. Example, guitar 1/channel one. Record take, channel two guitar processed. Play track one, use the interface channel one out to run it through post processor (say DBX), DBX out to interface in put 2 and record track two for the post processed recording.
If you want to stay analog for most of your recording. Then I suggest keep your setup analog and just do your mix down into the DAW. Use the DAW for converting to digital or use it for mastering. Otherwise, you are really making your digital journey really complex. But what you want to do can be done, just have to think through what it is you want to do.