[1] I've tried lineout of interface to amp guitar input but need a reamp as there's impedance mismatch
[2] Also tried headphone output from pc interface to line in on amp, better, but I got worried that since is an ampted headphone output it may damage the amplifier.
[2a] Also tried lineout to linein of amp but the volume is extremely low.
[3] Any option?
[3a] did I damage my amp by using an amped headhpone output to the line in?
It's impossible to answer your questions for sure because neither the line-in of your guitar amp nor the line-out of your PC interface are defined.
1. The actual guitar input on your amp has a very high impedance ("Z"), probably 1M Ohm or so, while the line out of your interface will have a far lower "Z". This by itself is not a problem, high Z output into a low Z input is a problem but not the other way around. However, a guitar input would expect a far lower voltage than a pc interface would output and that could cause a problem/damage, depending on the design of the amp and the output levels you are feeding it.
2. The "Line-in" input on guitar amps don't always mean the same thing. Sometimes it's for inputting a signal from a consumer device, such as a CD player (so the guitarist can play along with a recording), sometimes it's for plugging in another instrument, sometimes it's for plugging in a pro audio signal (from say a mixing desk or effects processor). A consumer device (such as a CD player) would have a nominal line level of -10dBv, instrument level is highly variable but typically would be lower or much lower than consumer line level, pro-audio line level is +4dBu which is the highest level. A headphone output would be lower than consumer line level and considerably lower than pro-audio line level but an amp'ed headphone output could of course be brought up to consumer or pro-audio line level depending on how much you amp'ed it.
2a. This would imply that the line in on your amp is pro-audio level and the line out of your pc interface is consumer level. However, that's not necessarily the case. It might just be that the line out of your PC interface is lower than nominal consumer level (which is common with different makes of consumer device) and the input of your guitar amp is consumer level.
3. I'm not really sure what you're trying to achieve? For starters, the output of a guitar amp is mono while the output of your pc interface is stereo and secondly, guitar amps are designed to very significantly colour the output.
3a. Almost certainly you haven't damaged your amp by using an amp'ed headphone output to your amp's line in input. It's more likely, though by no means certain, that you overloaded your amp to the point of damage by using the line out of your interface to the guitar input of your amp.
G
