Do I actually need to use a valve spring compressor to remove my valve stems? Is there a way to do it without using compressed air in pistons or a spring compressor?

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1990 190E 3.0L

yes, you need a compressor. the spring is nearly twice as long uncompressed and very strong. the locks are held in by the spring trying to uncompress.

i suppose if you had a compressor but difdn't have air, you could change the seals. i'd worry about dropping a valve unless you turned the motor by hand to tdc for each pair of pistons. engine tdc for 1 and 6, 120 degrees for 2 and 5, another 120 for 3 and 4. good luck, chuck.

Yes, you definitely need a valve spring compressor to get the locks and springs off.
But, to hold the valves from falling down you can use the shade-tree method of pushing a piece of rope or cord into the cylinder. Get a 3 ft piece of lawn mower starter cord or similar, turn the engine until the piston is down, and push the cord into the spark plug hole, leaving a little bit hanging out. Then turn the engine to move the piston up and compress the cord/rope against the head and the valves. This holds the valves in place quite well. Repeat on all the cylinders.

DG